the role of torsional modes in the electronic absorption spectrum of acetone

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Journal of Quantitative Spectroscopy &Radiative Transfer

Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 1553–1565

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The role of torsional modes in the electronic absorption spectrumof acetone

Aparna Shastri n, Param Jeet Singh, B.N. Raja Sekhar, R. D’Souza, B.N. Jagatap

Atomic and Molecular Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

a r t i c l e i n f o

Article history:

Received 17 February 2012

Received in revised form

15 March 2012

Accepted 19 March 2012Available online 30 March 2012

Keywords:

Acetone

Vacuum ultraviolet

Torsional modes

Synchrotron radiation

Vibronic assignments

Electronic spectrum

73/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.jqsrt.2012.03.019

esponding author. Tel.: þ91 22 25590343;

1 22 25502652.

ail address: [email protected] (A. Shastri).

a b s t r a c t

The electronic absorption spectrum of acetone is revisited to evaluate the role of hot bands

due to low lying torsional modes in the assignment of vibronic transitions. The UV–VUV

photoabsorption spectrum of acetone is recorded in the energy region 3.5–11.8 eV at a

resolution of �4 meV at 4 eV and �10 meV at 10 eV using synchrotron radiation. The

absorption spectrum is dominated by richly structured Rydberg series (ns, np and nd)

converging to the first ionization potential of acetone at 9.708 eV. Careful consideration of

hot band contributions from torsional modes and symmetry selection rules have resulted

in an improved set of vibronic assignments as compared to earlier room temperature work.

Revised quantum defect values for some of the Rydberg transitions and a few new

assignments in the nd series are also reported in this paper.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Acetone [(CH3)2CO] is the simplest aliphatic ketoneand is the first 10 atom molecule reported in the inter-stellar medium [1,2]. It also happens to be the smallestcarbonyl compound having two methyl (CH3) torsionalgroups [3]. Torsional modes have been recognized to playan important role in the structure and dynamics ofexcited electronic states of polyatomic molecules [3–10].In acetone, these vibrations have been explored with theaim of gaining insight into the nature of potential barriersinhibiting internal rotation [11–13]. Steric effects dueto the methyl groups also have a direct bearing onrotational–vibrational interactions in intramolecularvibrational relaxation and in determining structures ofstable conformers [13]. The nature and extent of thiscoupling in acetone is important in issues such as

ll rights reserved.

understanding of hyper-conjugative interaction with thecarbonyl bond, and of differences between these interac-tions in the ground and excited states [9]. Of the 24normal modes of acetone, the two torsional modes arewell separated in frequency from the other normal modes[14]. Due to the low frequency (77.8 and 124.5 cm�1) ofthese modes, an appreciable population is expected inexcited torsional levels at room temperature. For instance�70% of the ground state population would be in the n12

(a2) state and �55% of the ground state population isexpected to be in the n24 (b1) state at room temperature.Thus the probability of occurrence of intense hot bands inthe electronic absorption spectrum of acetone is ratherhigh. Despite the significant role played by the torsionalmodes in the room temperature photoabsorption spec-trum, very few reports have touched upon this aspect[6,9,15]. Recently, the complete room temperature elec-tronic absorption spectrum of acetone in the region up toits first ionization potential (9.708 eV) has been reportedby Nobre et al. [16]. Synchrotron radiation has been usedand the authors have proposed vibronic assignments for anumber of Rydberg and valence transitions, together with

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A. Shastri et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 1553–15651554

higher members of the ns, np and nd Rydberg series forthe first time. However, the assignments of some of thevibronic features appear to be doubtful. This, in ouropinion is partly due to non-inclusion of hot bandscorresponding to population of the torsional modes inthe ground state and may be partly due to an existingconfusion related to the numbering of the vibrationalmodes n13 to n24 of acetone. The sequence bands n12(1,1)and n24(1,1) are very close to the origin of each electronictransition and in the room temperature spectra they areblended with the origin band. Therefore, the n00 absorp-tion band is further broadened and the true position of n00

is not well defined. The assignments of the vibrationless(n00) Rydberg transitions and quantum defects in thiswork also show several discrepancies. The present studyis motivated by the need to clarify the assignments of theelectronic and vibronic transitions observed in the photo-absorption spectrum of acetone.

A review of published literature on acetone shows thatthe excited electronic states of acetone have been exten-sively studied experimentally [6,8–10,15–51] and theore-tically [34,52–64] in the past few decades with anobjective of understanding its complex photochemistryand internal rotational potential functions. The electronicabsorption spectrum of acetone lies largely in the VUVregion, with one weak absorption system in the UVregion. Subsequent to the early experiments by Duncan[17] and Lake and Harrison [18], spectroscopy of theexcited states of acetone has been revisited several times[7–10,15,16,19–51]. The lowest observed transition in theelectronic absorption spectrum is a broad band at about4 eV which is attributed to a dipole forbidden transition tothe first singlet excited state, viz. S1(n,pn) [16,19,20]. Thenext few valence states and Rydberg series converging tothe first ionization limit of acetone appear in the energyregion 6–9.7 eV. Considerable overlap and interactionbetween valence and Rydberg states lead to difficultiesin assignments.

In the VUV region, the spectrum is dominated byRydberg transitions [15,16,21–34] (ns, np and nd series),although a few valence transitions also occur and aremanifested through their interactions with the Rydbergstates [34]. Of these, the ns Rydberg series is the mostextensively studied and several works have focused onthe ny-3s transition lying at 6.35 eV [9,15,21–26].Absorption spectra of the 3s state of acetone at roomtemperature as well as low temperatures have beenstudied and some of the features assigned to excited statevibrational modes [15,22]. Polarization selected 2-photonand 3-photon resonance enhanced multiphoton ioniza-tion (REMPI) techniques have been used to investigate the3p Rydberg states of acetone and to assign the accom-panying vibronic structure [27–34]. The order of the3px(21A2), 3py(21A1) and 3pz(21B2) states has been satis-factorily established [27,28,32,34]. Some of the highlightsof these studies are the observation of several totallysymmetric a1 modes in the excited 3p states, reduction infrequency of the two a1 symmetric CH stretching modesand interpretation of the vibrational perturbation ofCH mode in terms of interstate vibrational coupling.In contrast, studies of the 3d Rydberg states and higher

members (nZ4) of the ns, np and nd Rydberg are few andsparse. Merchan et al. [34] have performed polarizationselected photoaccoustic and resonance multiphoton ioni-zation measurements on static and jet cooled samples ofacetone and acetone-d6 along with ab initio calculationsof vertical excited energies, potential energy curves andinteractions between valence and Rydberg states. Anoma-lies in the observed intensities of the 3d Rydberg statesare attributed to coupling of these states with a shortlived p-pn valence excited state [34]. Photoelectronspectroscopy of excited states populated by two- orthree-photon excitation has been employed by ter Steegeet al. [21] to study the vibronic structure and dynamicsof 3d Rydberg states. They have reported the origins ofseveral np and nd Rydberg states and also the measuredand calculated values of vibrational frequencies inRydberg states.

The UV photodissociation of acetone in the ultravioletregion is regarded as a prototype for the Norrish type Ia-cleavage reaction and is an important source of OHradicals in the upper troposphere [65]. From this point ofview, there have been numerous studies of the electro-nically excited states of acetone relevant to atmosphericchemistry, viz. photodissociation dynamics and quantumyield measurements [35–44] and UV absorption crosssection measurements [45,46]. Photoionization [47],photoelectron spectroscopy [48] and electron energy lossexperiments [49–51] have also been used to probe theelectronic structure of acetone. Several quantum chemicalcalculations dealing with different aspects of the photo-absorption and photodissociation processes in acetonehave been reported in recent years [52–64].

In this paper we report UV and VUV photoabsorptionstudies of acetone using synchrotron radiation and adetailed analysis of the observed vibronic features withparticular emphasis on symmetry based selection rulesand contribution from hot bands. Revised assignmentshave been given for a number of transitions and assign-ments have been proposed for a few bands which werenot assigned earlier.

2. Experimental

VUV absorption experiments are performed at thePhotophysics beamline at the synchrotron radiation facil-ity Indus-1. Details of the beamline and experimentalsetup have been discussed in earlier publications [66,67].Briefly, a 1-meter Seya–Namioka monochromator is usedto disperse the synchrotron radiation which is made topass through a 25 cm length stainless steel absorption cellwith LiF windows. The resolving power of the monochro-mator is �103 which results in a resolution better than45 cm�1 in the VUV region. The sample is held in a glassvial and connected to the absorption cell through a gasintroduction system consisting of several Swagelok valvesand adapters. Commercially available acetone of statedpurity better than 99% is used. Volatile impurities areeliminated by a few freeze–pump–thaw cycles. Thedetection system consists of a sodium salicylate coatedquartz window coupled to a UV–visible photomultiplier.The recorded signal is normalized at every step with

A. Shastri et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 1553–1565 1555

respect to the synchrotron beam current. The transmittedintensity through the evacuated cell (1�10�6 mbar)serves as the reference or I0 whereas the transmittedintensity through the sample filled in the cell is I. Theabsorption spectrum is generated using the Beer–Lambertlaw by plotting ln(I0/I) as a function of wavelength using astep size of 0.5 A. Absorption spectra are recorded atseveral sample pressures of 10�4 mbar to 1 mbar. Atomicabsorption lines of Xenon are used as wavelength stan-dards for calibration of the spectrum.

Fig. 2. The VUV absorption spectrum of acetone recorded using the

photophysics beamline at Indus-1. ns, np and nd Rydberg series

converging to the first ionization limit are marked.

3. Results and discussions

Photoabsorption spectra of acetone in energy andwavenumber scale are shown in Figs. 1–4. The VUVabsorption spectra of acetone observed in the presentwork are in good agreement with earlier works withrespect to energy positions and relative intensities[15,16,21,27,34].

Acetone belongs to the C2V point group with theground state electronic configuration being given byy..(2b1)2(5b2)2: 11A1. The highest occupied molecularorbital (HOMO) is a non-bonding lone pair orbital ny

orbital which can be approximately represented by the2py atomic orbital on oxygen. The first few excited statesof acetone arise from promotion of an electron from thislone pair orbital [21,34]. The symmetries and energies ofexcited electronic states 3s, 3px, 3py, 3pz, 3dxy, 3dyz and3dx2–y2 have been determined by experimental measure-ments as well as theoretical calculations [21,34]. A fewvibrational frequencies have been measured experimen-tally for the excited states 3s, 3px, 3py and 3pz

[9,15,16,21,22,27–29]. A summary of the excited statefrequencies available from some earlier studies is given inTable 1. It is known that for an allowed electronictransition, in the absence of vibronic coupling, only totallysymmetric vibrations are expected to be excited with

Fig. 1. Photoabsorption spectrum of acetone in the UV region showing

evidence of two overlapping peaks.

selection rules Dvi¼71, 72y. However due to vibroniccoupling (Herzberg–Teller effect), non-totally symmetricmodes can also be excited with weaker intensities. Hotsequence bands like n12(1,1) and n24(1,1) must also beincluded. A summary of the dipole allowed vibronictransitions (for Dvi¼71) under the C2V point group isgiven in Table 2.

In the following sections, assignments of valence andRydberg transitions observed in the present study arediscussed. It must be emphasized that there is sufficientevidence of hot band structure corresponding to themethyl torsional modes in the spectrum. It may also benoted here that while the 24 fundamental vibrationalmodes of acetone are well established from infraredspectroscopy [14], there exists some confusion in litera-ture regarding the classification and numbering of thenormal modes n13 to n24. The reason for this ambiguitycan be traced to the placement of the molecule withrespect to the Cartesian axes X,Y,Z. By convention, the Zaxis is taken to be the symmetry axis C2 which lies alongthe C¼O bond. If the molecule is assumed to be placed inthe YZ plane, so that the out of plane direction is along X,then the modes n13 to n19 belong to b2 and modes n19 ton24 belong to b1. Most of the recent works follow thisconvention. If, on the other hand, the molecule is assumedto be placed in the XZ plane with the out of planedirection along Y, then the normal modes n13 to n19

belong to b1 and modes n19 to n24 belong to b2 as givenin the compilation by Shimanouchi [14]. The torsionalmode corresponding to out of phase rotation of the twomethyl groups would be classified as b1 with the formerchoice of axes, and b2 with the latter. This mode has beenlabeled as n17 by some authors [9,15,22,27] and n24 byothers [14,16,21]. We follow the currently accepted con-vention of placing the molecule in the YZ plane whileretaining the numbering scheme followed by ter Steegeet al. [21] and Shimanouchi [14].

Fig. 3. VUV photoabsorption spectrum of acetone in the region of the 3p and 3d Rydberg states. Vibronic assignments of the prominent bands

are marked.

Fig. 4. VUV photoabsorption spectrum of acetone in the region of the 5s, 4p, 4d, 5p and 5d Rydberg states. Vibronic assignments of the prominent bands

are marked.


Table 1Vibrational frequencies of acetone in the ground, 3s, 3px, 3py, 3pz and ionic states.

Mode Ground Statea 3sb 3px 3py 3pz Ionic (Calc)c

n1 (a1) CH3-d-stretch 3019 3128b 3148b 3084

n2 (a1) CH3-s-stretch 2937 2424b 2940

n3 (a1) CO stretch 1731 1468c 1570

n4 (a1) CH3-d-deform 1435 1232 1373b/1395c 1367b/1376c 1395

n5 (a1) CH3-s-deform 1364 1191/1160 1293b 1309b 1272b/1280c 1269

n6 (a1) CH3 rock 1066 1047/1016 1056b/1050c 1023b/1058c 1036b/1042c 1045

n7 (a1) CC stretch 777 763 733b 675

n8 (a1) CCC deform 385 322/309 327b/326.2c 327b/324c 310b/316c 321

n9 (a2) CH3-d-stretch 2963 3007

n10 (a2) CH3-d-deform 1426 1432c 1400/1484

n11 (a2) CH3 rock 877 892c 868

n12 (a2) CH3 torsion (in phase) 77.8 118/133 72b/70.8c 70b�49/ 66.7d

n13 (b2) CH3-d-stretch 3019 3083

n14 (b2) CH3-d-stretch 2937 2933

n15 (b2) CH3-d-deform 1410 1385

n16 (b2) CH3-s-deform 1364 1313

n17 (b2) CC stretch 1216 984

n18 (b2) CH3 rock 891 868c 882

n19 (b2) CO in-plane bend 530 366c 350

n20 (b1) CH3-d-stretch 2972 3014

n21 (b1) CH3-d-deform 1454 1424

n22 (b1) CH3 rock 1091 1032

n23 (b1) CO out-of-plane bend 484 325b/453c 330b/484c 454

n24 (b1) CH3 torsion (out of phase) 124.5 175/215 129b 116/125d

a Ref. [14].b Ref. [28].c Ref. [21].d Ref. [11].

Table 2Electric dipole allowed (O) and forbidden (� ) vibronic transitions.

Symmetry ofnormal mode

Symmetry of excited electronic state

A1

(3py,3dyz)

A2

(3px)

B1

(3dxy)

B2

(3s,3pz, 3dx2�y2 )

n1 (a1) O � O On2 (a1) O � O On3 (a1) O � O On4 (a1) O � O On5 (a1) O � O On6 (a1) O � O On7 (a1) O � O On8 (a1) O � O On9 (a2) � O O On10 (a2) � O O On11 (a2) � O O On12 (a2) � O O On13 (b2) O O � On14 (b2) O O � On15 (b2) O O � On16 (b2) O O � On17 (b2) O O � On18 (b2) O O � On19 (b2) O O � On20 (b1) O O O �

n21 (b1) O O O �

n22 (b1) O O O �

n23 (b1) O O O �

n24 (b1) O O O �


3.1. Valence excited states

The first singlet excited state 11A2 (1A00) of acetone isdipole forbidden and corresponds to the n–pn electronic

transition. Ab initio studies show that the intensity forthis transition is largely borrowed from the allowed1A1–1B2 (n-3s) transition due to vibronic coupling throughthe n20, n22 and n23 normal modes and to a lesser extentfrom the 1A1–1B1 (n�3dxy) transition through the n19

mode [56]. This vibronic coupling is reflected by a changein molecular symmetry from C2V to Cs. Fluorescencestudies by Baba et al. have located the n00 transition ofthis electronic system at 30,435 cm�1 [10].

We have recorded the absorption spectrum of thissystem in the 3.6 to 5.8 eV region at relatively higherpressures (1–100 mbar) in order to obtain a good absorp-tion intensity (cf. Fig. 1). Whereas Nobre et al. [16] havereported some discrete structures superimposed on thecontinuum, in our spectrum no discrete features arediscernible. However the overall absorption at high pres-sure appears to be composed of two overlapping absorp-tion features as evident by the double maximum.Theoretical calculations by Merchan et al. [34] predicttwo valence transitions in this region of which one is asinglet (11A2) and the other is a triplet (13A2), bothcorresponding to excitation of ny-pn observed earlier inelectron impact studies [50] at 4.38 eV and 4.16 eVrespectively. A deconvolution of the observed peak intotwo Gaussians yields two peaks, one with high intensityand another with low intensity located at �4.64 and4.25 eV. Since the energy separation observed in thepresent work is considerably larger than the predictedvalue, therefore the weaker structure cannot be attributedto the triplet state. Higher valence states have not beendirectly observed so far, however experimental and the-oretical studies indicate that these states mix with


Rydberg states through vibronic interactions giving rise tothe observed anomalous intensities of the Rydberg states[20,34].

3.2. Rydberg states

In the energy region 6.35–9.7 eV extensive Rydbergseries converging to the lowest ionization limit of acetoneat 9.708 eV are observed (cf. Fig. 2). Each series can befitted to the well known Rydberg formula: En¼ I�R/(n�d)2where En is the energy of the transition, I is theionization potential, R is the Rydberg constant, n is theprincipal quantum number and d is the quantum defect.From the selection rule Dl¼71, transitions to ns and ndare expected to be relatively strong and transitions to npshould be weak with the intensities of higher Rydbergseries members expected to decrease as 1/n3 [68].

The various Rydberg series observed in the present studyare listed in Table 3, along with earlier results for compar-ison. In the nd series, 5dxy and 7dx2�y2 are not observed inthe present work, probably because they are merged withmore intense bands and cannot be distinguished within theexperimental resolution. However the expected positions ofthese transitions have been marked in Fig. 2. Accompanyingvibrational excitations are observed for several of the lowerRydberg series members. Vibrational frequencies for a fewmodes in the excited electronic states 3s, 3px, 3py and 3pz

have been measured by earlier workers [9,13,15,27–29]

Table 3Peak positions (in cm�1) and quantum defects (Q.D) of the n00 transitions of o

Present Nobre et al. [16] Present

n ns Q.D ns Q.D npy Q.D

3 51,221 0.987 51,337 0.983 59,688 0.572

4 66,244 0.983 66,291 0.977 69,045 0.557

5 71,364n 1.022 71,299 1.041 72,710 0.569

6 73,964 0.969 73,880 1.017 74,646 0.520

7 75,237n 1.015 75,243 1.009 75,682n 0.526

8 76,054n 1.010 76,050 1.018 76,326n 0.556

9 76,537n 1.111 76,566 1.045 76,815n 0.404

10 76,927n 1.061 76,953 0.974 77,068n 0.563

11 77,226 0.893 77,187 1.071

12 77,380 1.080 77,429 0.776

13 77,549 0.914 77,591 0.566

14 77,658 0.929 77,671 0.793

15 77,728 1.152 77,752 0.855

16 – – 77,833 0.684

Present Nobre et al. [16] Present

n ndyz Q.D ndyz Q.D ndx2�y2 Q.D

3 62,224 0.387 62,226 0.387 65,255 0.100

4 69,943n 0.376 70,170 0.326 71,174n 0.079

5 73,190 0.366 73,397 0.269 73,718n 0.106

6 74,909 0.312 74,937 0.288 75,168 0.081

7 75,816 0.354 75,905 0.232 – –

8 – – 76,453 0.292 76,537n 0.111

9 – – 76,865 0.257 76,922n 0.077

10 – – 77,107 0.412 – –

11 77,378 0.092

12 – –

n Denotes blended lines.

who have reported only the totally symmetric a1 vibrationsand torsional vibrations. We have used these measuredvalues wherever available for our assignments (cf. Table 1).For higher Rydberg series members and modes for whichexperimental measurements in excited electronic states arenot available, calculated ionic frequencies have been con-sidered following the work of ter Steege et al. [21]. This is areasonable assumption as vibrations in higher excited statesof the neutral molecule are expected to be closer to that ofthe ion. It may be noted that the frequencies of the n8 andn23 modes are almost equal in the excited states, due to aconsiderable reduction in frequency of the n23 mode from484 cm�1 in the ground state to �330 cm�1 in excitedstates. Similarly the values of n5 and n15 in the py states arevery close. Thus these modes cannot be distinguished fromeach other unless one of them is dipole forbidden for aparticular transition. Moreover, since several fundamentalsand combination bands have very similar frequencies,accidental degeneracies greatly complicate the assignments.

3.2.1. Assignments of ns Rydberg series and accompanying

vibronic structure

The ns Rydberg series is characterized by a quantumdefect of �0.98–1.10. In the present study we haveobserved 15 distinct members of this series, whereas formost of the other series fewer members are observed.The location of the origin of the 3s Rydberg transition hasbeen reported at various positions by different authors

bserved Rydberg series.

Nobre et al. [16] Present Nobre et al. [16]

npy Q.D npz Q.D npz Q.D

59,685 0.572 60,105 0.544 60,121 0.543

68,960 0.572 69,241 0.520 69,348 0.499

72,832 0.520 72,900 0.492 72,856 0.510

74,606 0.550 74,715 0.468 74,679 0.495

75,671 0.540 75,682n 0.526 75,703 0.500

76,300 0.593 76,326n 0.556 76,365 0.471

76,776 0.515 76,815n 0.404 76,776 0.515

77,107 0.412 77,068n 0.563 77,107 0.412

77,308 0.483

77,510 0.217

77,607 0.422

– –

– –

77,849 0.413

Nobre et al. [16] Present Nobre et al. [16]

ndx2�y2 Q.D ndxy Q.D ndxy Q.D

65,250 0.100 65,810 0.036 65,855 0.031

71,146 0.084 71,364n 0.022 71,299 0.041

73,719 0.106 – – 73,856 0.031

75,171 0.078 75,237n 0.015 75,252 0.0005

75,977 0.127 76,050 0.017 76,050 0.017

76,542 0.100 76,570 0.036 76,566 0.045

76,921 0.080 76,935 0.035 76,921 0.080

77,187 0.071 – – 77,187 0.071

77,373 0.123

77,510 0.217

Table 4Vibronic bands observed in the 3s, 4s, 5s and 6s Rydberg transitions

(in cm�1).

Present study Nobre et al. [16]

Position Assignment Position Assignment

51,221 3s

51,260 3sþn012�n0012 51,256 n00 (valence)

51,336 3sþn012 51,337 3s

51,527 3sþn0851,917 3sþn07�n0012 51,917 n07 (valence)

51,977 3sþn0752,268 3sþn0652,311 3sþn05�n0012 52 305 n06 (valence)

52,407 3sþn05 52,409 n04 (valence)

52,451 3sþn0452,505 3sþn05�n012

– 52,749 n04þn08 (valence)?

– 53,071 n04þn07 (valence)

53,315 3sþn0653,495 3sþn04þn06 53,474 n04þn06 (valence)

53,594 3sþ2n05 53,587 2n04(valence)

53,946 3sþ2n04þn08�n0012 53,958 2n04þn08 (valence)?

54,003(d) 3sþ2n04þn0854,256(w) 3sþn01�n0012 54,281 2n04þn07 (valence)

54,329(w) 3sþn01– 54,644 2n04þn06 (valence)

– 54,765 3n04 (valence)

– 55,128 3n04þn08 (valence)?

– 55,491 3n04þn07 (valence)

66,244 4 s 66,291 4s

66,572 4sþn08 66,621 4sþn0866,805n 4sþ2n08�n0012

66,871 4sþ2n80

67,075n 4sþ3n08�n0024

– 67,186 4sþ3n0867,256(s) 4sþn0667,398n 4sþn04�n0012; n0567,463 4sþn04 ;4n08�n0012

– 67,509 4sþ4n0867,561 4sþn03�2n0012;4sþn016

67,803 4sþ5n08�n0012

67,831 4sþ5n0867,864(s) 4sþ5n08– 67,936 4sþ4n08þn019

68,040n 4sþ6n08�n0024

68,154(s) 4sþ6n08 68,154 4sþ6n0868,306 4sþ2n06– 68,476 4sþ7n08– 71,299 5 s

71,364n 5s

– 71,509 y5sþn024

71,588 5sþ2n024

71,682n 5sþn08– 71,816 y5sþn024þn08– 71,945 5sþn024þn019

72,004n 5sþ2n08– 72,106 y5sþn024þ2n08– 72,332 y5sþn024þ2n016

72,388n 5sþn0672,659n 5sþ4n08 ; n05 72,541 5sþn024þ2n08þn019

73,718n 6s�2n0024

73,964 6s 73,880 6s

74,461n 5sþn0175,022 6sþn00675,451 6sþn010

75,237 7s

76,698n 7sþn010

n Denotes blended lines.y Forbidden by vibronic selection rules.


(51,203 cm�1 by Philis and Goodman [9], 51,232 cm�1 byMcDiarmid et al. [15], 51268.9 cm�1 by Gaines et al. [22],51,199 cm�1 by Cornish and Baer [24], 51204.5 cm�1 byMejıa-Ospino et al. [25], 51,337 cm�1 by Nobre et al. [16]).The ambiguity is due to sequence band structure arising frompopulation of the methyl torsional modes. In the presentroom temperature work, we assign the band observed at51,221 cm�1 to the 3s (n00) transition in agreement withseveral earlier works [9,15,25]. This assignment supports asatisfactory assignment of the rest of the vibrational structurebased on available upper state vibrational frequencies.

Nobre et al. [16] assign all vibronic structure observed inthe 6.3 to 6.9 eV region to a valence state whose symmetryis not specified, whereas the 3s Rydberg state is notassigned any vibrational structure by them. However, intheir work the higher Rydberg series members like 4s and5s have been assigned vibronic structure. While a few earlyworks [22,49] do refer to some ambiguity in the valence/Rydberg character of the transition at 6.35 eV, later theore-tical [34] and experimental works [9,15,22] clearly concludethat this state is Rydberg in nature, arising from thepromotion of a lone pair electron to the 3s Rydberg orbitalgiving rise to a 1B2 state. In fact, theoretical calculations donot predict any valence state in this energy region. Wetherefore conclude that all vibronic bands observed in thisenergy region belong to the 3s Rydberg state. Further, mostof these bands can be assigned satisfactorily provided hotband contributions from excited torsional levels are ade-quately considered. The vibronic assignments for the lowlying ns series members are listed in Table 4. The 4s andhigher members (cf. Figs. 3 and 4) lie above 8.2 eV andoverlap with the np and nd series. A unique feature of the 3stransition is that there is a considerable energy gap betweenthe 3s Rydberg state and the next higher members, there-fore vibronic structure appearing with 3s can be clearlydistinguished and is free from overlap with other transi-tions. The 4s, 5s and 6s Rydberg states are also accompaniedby distinct vibrational features, whereas in higher members(nZ7), vibrational bands are not observed. Vibrationalmodes excited in the 3s state are n1, n4, n5, n6, n7 and n8

whereas the vibrational features observed along with the 4sRydberg state are n3, n4, n5, n6 and n8. Hot bands due to thetorsional modes n12 and n24 are also seen along with a fewovertones and combination bands of the above modes. Inthe 5s vibronic structure, prominent bands are due to n8 andits overtones. Some contribution from n1 and n5 are alsoseen. It may be noted that the 5s assignments made byNobre et al. [16] to vibronic features involving n24 (cf.Table 4) are likely to be in error, as odd quanta of vibrationalexcitations with b1 symmetry would be forbidden in anelectronic transition to a 1B2 state, even with vibroniccoupling. In the 6s state, excitation of the n6 mode is seen.The most striking feature of the present assignments is thatthere are several bands which show structures displaced tothe lower wavenumber side by approximately 78 cm�1 or125 cm�1 which correspond to thermal population of tor-sional levels in the ground state. The highest member of thens series that can be distinguished clearly in our experimentis n¼15. For higher members (n415), overlap with otherRydberg series makes it difficult to make unambiguousassignments within the resolution used. This could be the


reason why the work by Nobre et al. which is carried out ata similar resolution shows deviations in the calculatedquantum defect for higher Rydberg series members. Forexample the n¼16 member of the ns series is reported bythem to be at 9.65 eV [16]. This gives a quantum defect of0.684 which deviates sharply from the range of valuesexpected for ns series.

3.2.2. Assignments of np Rydberg series and accompanying

vibronic structure

The 3p Rydberg series have been extensively studiedearlier [13,21,27,28,34]. CASSCF calculations [34] predictthe symmetries of the three 3p Rydberg transitions to be21A2 (ny-3px), 21A1 (ny-3py) and 21B2 (ny-3pz). Thetheoretically predicted symmetries and energies are wellcorroborated by experiments [21,32,34], establishingthe n00 transitions of the 3px, 3py and 3pz series to be at7.36, 7.41 and 7.45 eV respectively. The 3px(1A2) Rydbergtransition is dipole forbidden and is not expected toappear in the photoabsorption spectra. Nobre et al. [16]

Table 5Vibronic bands observed in the 3py, 4py, 5py and 6py Rydberg transitions (in c


Position Assignment Position

59,688 3py 59,685

– 60,008

– 60,120

60,252n 3pyþ2n019�2n0012 60,258

60,410n 3pyþ2n019

– 60,572

60,775 3pyþ3n019

–

– 60,903

– 61,056

61,127n 3pyþ4n190

–

61,382(s) 3pyþ5n019�n0012

– 61,459

– 61,879

62,128 3pyþn02 62,153

– 62,226

62,446(s) 3pyþn02þn08– 62,556

–

– 63,395

– 68,960

69,045 4py

– 69,928

70,082 4pyþn06– 70,896

– 71,945

72,710(s) 5py 71,832

73,661n 5pyþ2n023

74,646(s) 6py 74,606

75,331 6pyþn0775,682n 6pyþn06;7py 75,671

76,207 6pyþn0376,320n 8py 76,300

76,698n 8pyþn019 ;6pyþ2n06

n Denotes blended lines.# REMPI study of jet cooled sample.

have attributed a few absorption peaks to vibrationalstructure accompanying the 3px transition, however theseassignments are mostly to the n4 (a1) mode which is againdipole forbidden. Only modes of a2, b1 or b2 symmetrywould be allowed by vibronic selection rules in this case.In the present study only the npy and npz Rydberg serieshave been observed. Measured quantum defects lie in therange of �0.4 to 0.6 (cf. Table 3). The oscillator strength ofthe 3py state is predicted to be an order lower than thatof the 3pz state, and both 3p states are expected tohave considerably lower intensities than the 3s state [34].As seen from Fig. 2, the assignment of the features at59,695 cm�1 and 60,107 cm�1 to the n00 bands of 3py and3pz are consistent with this prediction.

Assignments of the vibrational bands accompanyingthe 3py Rydberg state (cf. Fig. 3) are listed in Table 5. The3pyþn19 band is not clearly resolved as its expectedposition coincides with the rapidly rising part of 3pz.Several overtones of the n19 mode are however observedclearly. While Nobre et al. [16] also assign several peaks to

m�1).

Xing et al. [28]#

Assignment Position Assignment

3py 59,769 3py

3pyþn083pyþn019 60,100 3pyþn083pyþ2n08

60,419 3pyþ2n083pyþ3n08;2n019

60,759 3pyþ3n0860,833 3pyþn06

3pyþ4n08; n043pyþ3n019 61,078 3pyþn05

61,382 3pyþn06þ2n08

3pyþ4n019

3pyþ5n019

3pyþ2n043pyþn02 62,193 3pyþn02

3pyþn02þn08 62,517 3pyþn02þn0862,897 3pyþn01

3pyþ3n044py

4pyþn06

4pyþ2n064pyþ3n065py

6py

7py

8py


overtones of n19, the measured frequency shows a largevariation in the range of 400–480 cm�1. In the presentassignment, the measured spacing between successiveovertones is almost constant with a value of �350–360 cm�1. Although there is no earlier measured value ofn19 in the 3py state available for comparison, the calculatedvalue of ionic frequency of n19 and measured value of n19 inthe 3pz state are in this range (cf. Table 1) [21]. As thefrequencies of the vibrational modes in Rydberg states areexpected to be closer to the ionic values than the groundstate values, our assignment seems reasonable.

The values of n2 and n4 in the 3py state have beentaken as �2541cm�1 and �1230 cm�1 in the work ofNobre et al. The experimentally measured value of n2 inthis state is 2424 cm�1 [28]. We have observed this bandat �2440 cm�1. For the n4 mode on the other hand noprior measurements exist in the 3py state. However thefrequency of this mode has been measured to be �1367–1395 cm�1 in the 3px and 3pz states (cf. Table 1). We haveobserved the n4 excitation at �1370 cm�1. Therefore,it can be concluded that our observations are consistent

Table 6Vibronic bands observed in the 3pz, 4pz, 5pz and 6pz Rydberg transitions (in cm

Present study Nobre et al. [1

Position Assignment Position

60,105 3pz 60,120

60,252n 3pzþn008�2n0012

60,410n 3pzþn08 60,419

60,674(d) 3pzþ2n08�n0012

– 60,750

–

60,891 3pzþ3n08�2n0012; n06�2n0024

– 61,056

61,127n 3pzþn06;4n08�2n0024 61,145

–

61,382n 3pzþn05;4n08 61,387

61,693 3pzþ5n08 61,726

– 62,153

63,060 3pzþn0263,250n 3pzþn01– 63,798

66,426(s) 3pzþ2n0169,241 4pz

– 69,348

– 69,686

69,560(s) 4pzþ2n0869,943n 4pzþn07;2n019 69,928

– 70,049

70,145(s) 4pzþ3n08 70,170

70,271n 4pzþn06;3n019

70,503 4pzþ4n08 70,493

70,788(br) 4pzþ5n08 70,815

71,174n 4pzþ6n08 71,138

71,482n 4pzþ7n0872,193n 4pzþn0272,900 5pz 72,856

– 73,074

73,218n 5pzþn0873,661n 5pzþn07

n Denotes blended lines.# REMPI study of jet cooled sample.y Forbidden by vibronic selection rules.

with the earlier reported measurements of these frequen-cies [21and references therein]. Other modes observedalong with the npy series are the totally symmetric modesn2, n3, n6 and n7 and 2n23. The possibility of the 3py (1A1)state being perturbed by the valence state p-pn of thesame symmetry has been pointed out in earlier theore-tical work [34] and experimental evidence for the samehas also been reported [30].

Vibronic assignments in the npz series, some of whichare marked in Figs. 3 and 4 are given in Table 6. The 3pz

and 4pz Rydberg states are rich in vibrational structurecorresponding to the n8 mode and its overtones, in overallagreement with the observations of Nobre et al. [16].In addition we have observed several bands that arise dueto the ground state torsional modes and have beenaccordingly assigned as hot bands. Other modes excitedin this series are n1, n2, n5, n6, n7 and n19. Vibrationalstructure is observed up to n¼5, after which it dies out.The assignment of the band at 9.06 eV by Nobre et al. to5pzþn24 violates the symmetry based selection rules asexplained earlier in the case of the 5s bands.

�1).

6] Xing et al. [28]#

Assignment Position Assignment

3pz 60,100 3pz

3pzþn08 60,410 3pzþn08

3pzþ2n08 60,733 3pzþ2n0860,833 3pzþn07

3pzþ3n083pzþn06 61,078 3pzþn063pzþn043pzþ4n08 61,372 3pzþn053pzþ5n083pzþ2n06

63,248 3pzþn013pzþ3n04

4pz

4pzþn08

4pzþ2n084pzþn08þn019

4pzþ3n08

4pzþ4n084pzþ5n084pzþ6n08

5pzy5pzþn024

Table 7Vibronic bands observed in the 3dyz and 4dyz Rydberg transitions

(in cm�1).



62,224 3dyz 62,226 3dyz

62,539 3dyzþn08 62,556 3dyzþn0862,911 3dyzþn07 62,935 3dyzþ2n0863,060 3dyzþn011

63,250* 3dyzþn022; n06 63,274 3dyzþ3n08– 63,476 3dyzþn0463,520 3dyzþn016

– 63,597 3dyzþ4n0863,888 3dyzþ2n011�n0012 63,895 3dyzþ5n0864,140 3dyzþ6n8

0

– 64,202 3dyzþ6n0864,298 3dyzþ2n022 ;2n0664,444 3dyzþ7n08 64,516 3dyzþ7n08– 64,686 3dyzþ2n0464,864 3dyzþ2n016 64,766 3dyzþn0265,173 3dyzþn0269,943* 4dyz

– 70,170 4dyz

70,271* 4dyzþn019; n0870,576 4dyzþ2n08– 70,654 4dyzþn019

– 71,630 4dyzþn019þn06

n Denotes blended lines.


For n46, the two components of the series npy andnpz are not resolved in our experiment and the two seriesmerge into a single one. The highest principal quantumnumber observed in our work is 10. Nobre et al. [16] havereported the npy series up to n¼10, and the npz series upto n¼16. However there seems to be some discrepancy inthe npz series assignments, as they have not observedn¼14 and n¼15, but have nevertheless assigned the bandat 9.652 eV to n¼16. Moreover it is somewhat surprisingthat they could distinguish the 16s band at 9.65 eV fromthe 16pz band at 9.652 eV with their stated resolution of6 meV at 10 eV.

3.2.3. Assignments of nd Rydberg series and accompanying

vibronic structure

Experimental observation of the nd Rydberg series upto high values of n were reported for the first time byNobre et al. [16], with earlier studies being limited up tothe 7d state [21]. Theoretical calculations predict that ofthe five possible components of the 3d Rydberg state(1A13dyz,

1B23dx2�y2 , 1A23dxz, 1B23dz2 and 1B13dxy), 3dxz isforbidden and 3dz2 has very low oscillator strength.Consequently, only three series should be observed. Ofthese only two have been observed by earlier workers[21,34] whereas Nobre et al. [16] have reported all three,extending up to n410. A possible reason for this could bethat the earlier works were carried out using the REMPItechnique which is capable of detecting only those stateswhich live long enough to absorb additional photons forionization [34]. Thus the short lived diffuse state(1A1 3dyz) was not detected by them. In our work, allthree series components reported by Nobre et al. areobserved. While the overall spectral features are in goodagreement with their work, revised assignments havebeen proposed for some of the bands.

Note that expected relative intensity is predicted to bemaximum for 3dyz, followed by 3dx2-y2 which is roughly1/6th of 3dyz, followed by 3dxy which is about 1/3rdof3dx2-y2 . In the photoabsorption spectrum however, theintensities of 3dxy and 3dx2�y2 are comparable to oneanother and are much more than that of 3dyz. Thisanomalous intensity pattern has been explained by Mer-chan et al. as arising due to a mixing between the (1A1)3dyz Rydberg state and the hitherto unobserved (1A1)p–pn valence state [34]. Ter Steege et al. [21] have alsopointed out that this vibronic coupling may be respon-sible for loss of intensity of Rydberg states of A1 symme-try. We concur with this interpretation and observe thatthis anomaly appears to be maximum for the n¼3 states,with intensities of all the three 4d components beingalmost equal.

The positions of the Rydberg series members andquantum defects obtained in the present work differ fromthose of Nobre et al. [16] on several counts (cf. Table 3). Inthe ndyz series, we report Rydberg members up to n¼7,whereas Nobre et al. have reported up to n¼10. Howeverthe variation in quantum defect over the series is quitelarge in the earlier work, whereas the present assignmentshows almost constant quantum defect. A similar trendcan be seen in the ndx2-y2 series, which is the mostextensive. In the present experiment the n¼7 and n¼10

states have not been observed. This could be due toblending with other bands and limitations imposed byexperimental resolution. In this context we would like topoint out that the assignment by Nobre et al. of the bandat 9.76 eV to 37dx2�y2 is certainly incorrect, as their statedexperimental resolution of 6 meV would succeed in justresolving the 16dx2�y2 and 17dx2-y2 transitions. In the ndxy

series, the quantum defect reduces from a value of 0.035for 3dxy till it reaches a value of 0.015 for n¼6 after whichit again increases to �0.035. Quantum defect values givenby Nobre et al. show considerably larger deviations [16].The smaller deviation observed by us can be attributed tothe effect of perturbations from nearby valence states.Further understanding of these anomalies would benefitfrom complementary techniques like high resolutionphotoelectron spectroscopy.

Vibronic assignments carried out for the ndyz,ndx2-y2 and ndxy series are listed in Tables 7–9. The bandat 9.22 eV assigned by Nobre et al. (cf. Table 9) to5dxyþn19 is vibronically forbidden. Moreover, the fre-quency of n19 in the Rydberg state is likely to be reducedfrom its ground state value, hence this assignment isdoubtful. In our work, vibrational structure is observedonly for n¼3 and 4 in all three series. Some of theassignments are marked in Figs. 3 and 4. Extensiveprogressions corresponding to excitation of the n8 modeare seen in all the three series along with a few othermodes (n1, n2, n3, n5, n6, n7, n11, n16, n19 and n22). A uniquefeature of the 3d series is the appearance of the n11 modein all the components. Vibrational assignments for the3dxy and 4dxy states are reported here for the first time.

Table 8Vibronic bands observed in the 3dx2-y2 and 4dx2-y2 Rydberg transitions

(in cm�1).

Present Nobre et al. [16]


65,255# 3dx2�y2 65,250 3dx2�y2

65,553 3dx2�y2þn08 65,597 3dx2�y2þn0865,855 3dx2�y2þ2n0866,137 3dx2�y2þ3n08

66,186 3dx2�y2þ3n08 66,291 3dx2�y2þn0666,471n 3dx2�y2þ4n08 66,508 3dx2�y2þ4n0866,805n 3dx2�y2þn03;5n08 66,823 3dx2�y2þ5n0867,138(s) 3dx2d�y2þ6n08 67,138 3dx2�y2þ6n0868,330 3dx2�y2þn0171,174n 4dx2�y2 71,146 4dx2�y2

71,482n 4dx2�y2þn0872,004n 4dx2�y2þn011

72,193n 4dx2�y2þn0673,718n 5dx2�y2 73,719 5dx2�y2

74,250 4dx2�y2þn01

n Denotes blended lines.# Observed at 65,250 cm�1 by ter Steege et al. [21].

Table 9Vibronic bands observed in the 3dxy and 4dxy Rydberg transitions

(in cm�1).

Present Nobre et al. [16]


65,810# 3dxy 65,855 3dxy

66,114 3dxyþn0866,471n 3dxyþ2n0866,724 3dxyþn011

66,805n 3dxyþ3n0867,075n 3dxyþn0567,398n 3dxyþ5n0868,040n 3dxyþ7n0871,364n 4dxy 71,299 4dxy

71,682n 4dxyþn0872,004n 4dxyþ2n0872,388n 4dxyþn0672,659n 4dxyþn0573,434 4dxyþ2n06– 73,856 5dxy

– 74,155 5dxyþn08– 74,364 y5dxyþn019

74,461n 4dxyþn01 74,485 5dxyþ2n08– 74,768 5dxyþ3n08

n Denotes blended lines.# Observed at 65,910 cm�1 by ter Steege et al. [21].y Forbidden by vibronic selection rules.


4. Conclusions

The UV and VUV photoabsorption spectrum of acetonehas been recorded using synchrotron radiation. The onlyvalence transition observed lies in the UV region andcorresponds to the n–pn (1A2) dipole forbidden transitionwhich borrows intensity from the n�3s (1B2) transitionthrough vibronic coupling. The VUV absorption spectrumshows richly structured Rydberg series converging to the

first ionization potential of acetone at 9.708 eV. A detailedanalysis of the vibronic structure is carried out. The onlyearlier report of the photoabsorption spectrum of acetonein the entire UV–VUV region is by Nobre et al. [16], whichwhile providing a comprehensive electronic absorptionspectrum of acetone for the first time showed severaldiscrepancies and inconsistencies in assignments. In thepresent work, discrepancies in earlier works are sortedout and a few new assignments are given, particularly inthe nd Rydberg series. A major difference in the presentwork is that all symmetry based selection rules are takeninto account carefully while making assignments. Inclu-sion of hot band structure due to population of low lyingtorsional modes in the ground state has also resulted inclarification of some of the assignments.

The quantum defect values observed in the ns and npseries show fairly regular behavior, whereas quantumdefects observed in the nd states show considerablevariation between different sub-series. The most exten-sive vibronic structure free from overlap with othertransitions is seen in the 3s transition. In all ns transitions,only totally symmetric vibrations are observed, accom-panied by considerable hot band structure. In the npseries, in addition to a few totally symmetric modes, then19 mode appears along with several overtones. In the ndseries, in contrast we observe several non-totally sym-metric modes. This observation along with the anomalousquantum defects indicates a strong possibility of pertur-bation by some nearby state.

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the role of torsional modes in the electronic absorption spectrum of acetone

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