12 OPTICS LETTERS / Vol. 5, No. 1 / January 1980

Infrared-ultraviolet double resonance in D2CO vapor

Brian J. Orr and Gary F. NuttSchool of Chemistry, The University of New South Wales, Sydney, N.S.W. 2033, Australia

Received September 4,1979

Rotationally resolved infrared-ultraviolet double-resonance effects have been observed in D2CO vapor by usingpulsed radiation from a CO2 laser and a tunable dye laser. The infrared laser pumps D2CO molecules in the 14 rovi-brational band, and subsequent excitation by the dye laser in the 365-nm 410 vibronic band is detected by fluores-cence at 420 nm. The results consolidate assignments from high-resolution absorption spectra and yield estimatesof rotational-relaxation rates.

Double-resonance spectroscopic techniques, involvinga sequence of two one-photon interactions in a singleatom or molecule, have been applied extensively inspectrum characterization and in studies of molecularrelaxation and photochemical processes. A recent re-viewl has shown that, of the many combinations of ul-traviolet, visible, infrared, and radio-frequency radia-tion that can be used in double-resonance spectroscopy,relatively few examples of infrared-ultraviolet doubleresonance (IRUVDR) have been reported. Moreover,some of the reported observations of IRUVDR haveproved ambiguous in their interpretation; for example,apparently straightforward IRUVDR mechanisms inthe biacetyl molecule2' 3 have subsequently been shown4

to involve infrared multiphoton heating of moleculesoptically excited into the triplet state. This Letterdescribes an LRUVDR spectroscopic study of formal-dehyde-d 2 , D2CO, the interpretation of which does notsuffer from the previous ambiguities.

Our method is shown schematically in Fig. 1. Itconsists of infrared excitation (1 - 0) of a specific ro-tational-vibrational energy level (1) of D2CO, followedby promotion of molecules from that level to an elec-tronically excited level (2), which is monitored by de-tection of fluorescence (2 - 3). The source of infraredradiation is a pulsed, UV-preionized CO2 laser (typicaloutput pulse energy 0.5 J, pulse width r30 nsec FWHM,repetition rate 1 Hz), the wavelength of which is se-lectable by means of an intracavity diffraction grating.Ultraviolet radiation in the 365-nm region is generatedby a nitrogen-laser-pumped tunable dye laser (Molec-tron UV400/DL200, typical output pulse energy 150 ,J,pulse width -5 nsec FWHM, PBD dye in toluene-ethanol used). The timing of the two laser pulses iscontrolled by a delay circuit that enables the lasers tobe fired simultaneously or with a specific relative delay,subject to a jitter time of less than 200 nsec. The twolaser beams are weakly focused to beam waists of 10 and5 mm, respectively, and counterpropagate through aglass sample cell (length 30 cm, diameter 2 cm)equipped with stainless steel flanges holding NaCi en-trance and exit windows. D2CO vapor, produced byheating the corresponding polymer (supplied by Merck,Sharp and Dohme of Canada) in a sidearm, is intro-duced into the cell and its pressure registered by an

electronic manometer. Fluorescence excited by theIRUVDR process near the midpoint of the cell is viewedthrough a side window and detected in the 4 18-424-nmspectral region by a set of bandpass filters and a 1P28photomultiplier. The signals are processed either bya fast-storage oscilloscope (Tektronix 7633, 100-MHzbandwidth) or a signal-averaging system consisting ofa transient recorder (Biomation 610B) and multi-channel signal averager (Hewlett Packard 5480B). Thelatter provides background subtraction facilities andpermits simultaneous collection of fluorescence-timeprofiles and of fluorescence intensity integrated by anappropriate preamplifier (Molectron LP 143). For agiven CO2 laser wavelength, two types of experiment areparticularly relevant in this investigation: (a) with thetwo laser pulses synchronized, the dye-laser wavelengthis scanned to produce an ultraviolet excitation spec-trum; (b) with the dye laser tuned to a specific wave-

I I

EXCITEDELECTRONIC

- 2 STATE

FLUORESCENCE

3

ON

1 GROUNDELECTRONICSTATE

0

Fig. 1. Excitation scheme for infrared-ultraviolet doubleresonance (IRUVDR). Levels 0 and I are specific rota-tional-vibrational levels of the ground electronic state, andlevel 2 belongs to an excited vibronic state of the molecule.

0146-9592/80/010012-03$0.50/0 ©) 1980, Optical Society of America

January 1980 / Vol. 5, No. 1 / OPTICS LETTERS 13

length, the interval between the two laser pulses isvaried to provide kinetic information.

The molecule selected in this work, D2CO, is an idealcandidate for IRUVDR studies in a number of respects.First, its infrared absorption spectrum in the region ofCO2 laser emission has been adequately characterizedby both laser Stark 5 and conventional 6'7 spectroscopy;it consists of three fundamental rotational-vibrationalbands, V3 , V4 , and v6 , centered 6 at 1100.4, 938.0, and989.25 cm-1 , respectively. Of these, excitation in theV4 band is expected to enhance ultraviolet absorptionin the 365-nm 410 band of the A 1A 2 -X 1A, electronicsystem of D2CO, which has. also been thoroughly ana-lyzed.8 This ultraviolet excitation populates the v =

0 level (40) of the first excited singlet electronic state (A1A 2), the population of which can readily be monitoredby means of fluorescence in the 420-nm region, whichis remote from the ultraviolet-excitation wavelength.This 420-nm emission is known to consist 9 of two com-ponents, centered at 422.0 nm (450 band) and 419.5 nm(210430 band), to exhibit1 0' 11 a 4.5-pusec decay time in the

limit of zero pressure, and to be uncomplicated bycontributions from vibrationally excited levels of the A1A 2 state. In terms of the simplified scheme of Fig. 1,therefore, the levels labeled 0, 1, and 2 are rotationalsublevels of the 40, 41, and 40 vibronic states. Likewise,the level labeled 3 corresponds to the two states 45 and2143. An added impetus to studies of IRUVDR inD2 CO is the possibility of its relevance to mechanismsof optically pumped far-infrared and radio-frequencysources1 2 .and of isotopically selective photochemistryinvolving either ultraviolet 1 3 or CO2 (Ref. 14) lasers.

An example of the results obtained by this methodis shown in Fig. 2. D2 CO molecules are pumped in thisinstance by the 10.8-,um P(36) CO2 laser line (929.017cm-1 ), the frequency of which is believed (from ex-trapolation of laser Stark spectroscopic results5' 7) to lieapproximately 0.015 cm- 1 below that of the 81,7 ' 82,7

transition 1 5 in the v4 infrared absorption band of D2 CO.The levels 0 and 1 of Fig. 1 therefore correspond, re-spectively, to the 82,7 and 81,7 rotational sublevels of the40 and 41 vibronic states. Infrared pumping shouldtherefore enhance directly the ultraviolet absorptionof D2 CO on any vibronic transition of the 365-nm 410band that originates in the 81,7 rotational sublevel of the41 vibronic state. Analysis of the 410 band8 shuws thatthere are four such transitions of reasonable intensity,as follows:

A, 92,8 - 81,7 at 27381.650 cm- 1

B, 82,6 - 81,7 at 27366.947 cm- 1

C, 72,6 81,7 at 27351.998 cm- 1

D, 80,8 - 81,7 at 27350.498 cm- 1

(365.208 nm);(365.404 nm);(365.604 nm);(365.624 nmn).

These transitions, bearing the above labels, are indi-cated on the spectrogram 8 shown in Fig. 2(a) and arefound to correspond closely to the infrared-inducedfeatures that appear in Fig. 2(b). The prominent fea-ture labeled H that appears in both the IRUVDR trace[Fig. 2(b)] and the background spectrum [Fig. 2(c)] isthe intense central band head of the 410 band, whicharises from the equilibrium population of molecules inthe 41 vibronic state at sample temperature (-30'C).

A H B O CD

80 ~~~~~~(b)

, J . f / ~~~~~~~~(c)

365.2 365.4 365.6VACUUM WAVELENGTH (nm)

Fig. 2. (a) Portion of a high-resolution absorption spectro-gram of D2CO vapor in the vicinity of the 365-nm 410 bandorigin (labeled 0). (b) Excitation spectrum of D2CO vapor(pressure -5 mTorr), showing IRUVDR peaks produced bypumping with the 10.8-Am P(36) CO2 laser line. The dye-laser pulse was delayed by 0.8 ± 0.1 ,sec with respect to theCO2 laser pulse in obtaining this trace. (c) Background ex-citation spectrum (infrared laser off), corresponding to (b),that is due to thermal population of the 41 vibrational level.Assignment of the spectral features labeled A, B, C, D, and His discussed in the text. The wavelength scale applies pre-cisely only to traces (b) and (c), which were obtained with ascan rate of 0.5 nm min-' and an instrumental time constantof 3 sec.

This band head has been used to locate the wavelengthscale in Fig. 2. The relative vacuum wavelength in-tervals between the features A, H, B, and the centroidof C and D are predicted, 8 respectively, to be 0.085,0.110, and 0.215 nm. The corresponding intervalsmeasured from Fig. 2(b) are 0.080,0.110, and 0.220 nm(each ±0.01 nm). Further confirmation of the assign-ment of the infrared-induced features in Fig. 2(b) comesfrom their relative intensities, which are predicted8 tobe A:B:(C + D) = 1.0:1.7:2.1 and observed to be ap-proximately 1.0:1.9:1.9. The representative results ofFig. 2 therefore provide compelling spectroscopic evi-dence of a genuine rotationally resolved IRUVDRprocess.

The FWHM of the IRUVDR features in Fig. 2(b) is0.05 nm (3.5 cm-1 ), which is attributed principally tothe bandwidth of the dye laser (0.04 nm FWHM). Thisrelatively low resolution is sufficient to mask the sepa-ration between predicted features C and D in Fig. 2(b)and to obscure the appearance of collision-inducedsatellite structure in the IRUVDR spectrum. Colli-sion-induced effects are apparent, however, in experi-ments at higher sample pressures or when the delaytime between laser pulses is increased. The latter effectis illustrated in Fig. 3 for the 365.62-nm IRUVDR fea-ture comprising transitions C and D. The intensity of

14 OPTICS LETTERS / Vol. 5, No. 1 / January 1980

p(D2CO) = 5 mtorr

0.2nmII__

1.8 ps

results, with improved wavelength and time resolution.This preliminary report amply demonstrates the po-tential of the technique in two respects: the use of in-frared pumping to project a specific set of rotational-vibronic transitions, all sharing a common lower level,out of a rotationally congested vibronic spectrum; themeasurement of rotational relaxation rates for specificquantum states, using the high sensitivity and speci-ficity of the IRUVDR method.

Financial support from the Australian ResearchGrants Committee is gratefully acknowledged.

References

2.4 ps

Fig. 3. Effect of dye-laser delay time on the 365.62-nm (C+ D) IRUVDR feature of D2CO, obtained with the 10.8-AmP(36) CO2 laser line. The delay times between dye-laser pulseand CO2 laser pulse are as indicated on the figure. Otherconditions are: D2 CO pressure, 5 mTorr; time delay jitter,- ±0.2 ,usec; dye-laser scan rate, 1.4 nm min-'; time constant,3 sec. A background trace (infrared laser off) is shown as abroken curve immediately below the 2.4-,4sec trace.

the IRUVDR peak is seen to decay, and to broadenmarginally, with increasing delay time. This is at-tributed to rotational relaxation of the pumped 81,7rotational-vibrational population, which causes theeffects of infrared pumping to be distributed through-out the rotational manifold of the 41 vibrational stateafter an elapsed time of -5 gsec at a pressure of 5mTorr. Analysis of preliminary kinetic results placesan upper limit of 10 nsec Torr on the inverse rate con-stant for rotational relaxation of the 81,7 rotational-vibrational level. Such a relaxation rate is consistentwith experimentall6 "7 and theoreticall6 "8 results forH2CO and HDCO and for other molecules.'

Similar IRUVDR effects can be observed in D2COwith a number of other CO2 laser lines. For example,IRUVDR signals produced by irradiation of D2 CO withthe 10.55-,um P(16) CO2 laser line are consistent withits coincidence with the 123,10 122,10 rovibrationaltransition at 947.74 cm-' in the V4 band, which is a re-gion of the infrared spectrum heavily overlapped by theV6 band.7 Current experimental efforts are aimed atproviding more-extensive spectroscopic and kinetic

1. J. I. Steinfeld and P. L. Houston, "Double-resonancespectroscopy," in Laser and Coherence Spectroscopy, J.I. Steinfeld, ed. (Plenum, New York, 1978), pp. 1-123.

2. A. Yogev and Y Haas, Chem. Phys. Lett. 21, 544(1973).

3. B. J. Orr, Chem. Phys. Lett. 43, 446 (1976).4. I. Burak, T. J. Quelly, and J. I. Steinfeld, J. Chem. Phys.

70, 334 (1979).5. D. Coffey, C. Yamada, and E. Hirota, J. Mol. Spectrosc.

64, 98 (1977).6. T. Nakagawa, University of Tokyo, Tokyo, Japan, per-

sonal communication (1976).7. B. J. Orr and J. W. C. Johns, unpublished infrared ab-

sorption spectrum of D2CO vapor, recorded with a reso-lution of -0.05 cm-1 in the 9-11-gm spectral region.

8. B. J. Orr, Spectrochim. Acta 30A, 1275 (1975).9. J. C. D. Brand, J. Chem. Soc. 858 (1956).10. E. S. Yeung and C. B. Moore, J. Chem. Phys. 58, 3988

(1973).11. R. G. Miller and E. K. C. Lee, Chem. Phys. Lett. 41, 52

(1976); J. Chem. Phys. 68, 4448 (1978).12. D. Dangoisse, A. Deldalle, J. P. Splingard, and J. Bellet,

IEEE J. Quantum Electron. Q-E 13, 730 (1977).13. E. S. Yeung and C. B. Moore, Appl. Phys. Lett. 21, 109

(1972); R. V. Ambartsumyan, V. M. Apatin, V. S. Leto-khov, and V. I. Mishin, Sov. J. Quantum Electron. 5,191(1975); J. Marling, J. Chem. Phys. 66, 4200 (1977).

14. G. Koren, U. P. Oppenheim, D. Tal, M. Okon, and R. Weil,Appl. Phys. Lett. 29, 40 (1976); G. Koren, M. Okon, andU. P. Oppenheim, Opt. Commun. 22, 351 (1977); G. Korenand U. P. Oppenheim, "Multiphoton dissociation informaldehyde," in Laser-Induced Processes in Molecules,K. L. Kompa and S. D. Smith, eds. (Springer-Verlag,Berlin, 1979), pp. 209-212.

15. We use the notation JKa,Kc for the rotational levels of anear-prolate asymmetric rotor.

16. T. Oka, J. Chem. Phys. 47, 13 (1967); Adv. At. Mol. Phys.9, 127 (1973).

17. M. Takami and K. Shimoda, Jpn. J. Appl. Phys. 11, 1648(1972).

18. V. Prakash and J. E. Boggs, J. Chem. Phys. 57, 2599(1972).

0.6ps 1.1 Fs