Volume 89. number 6 CHEMICAL PHYSICS LETTERS 9 July 198’

ULTRAVIOLET EMISSION OF MOLECULAR CHLORINE

Takaslu ISHIWATA, lchiro FUJIWARA and Ih~uzo TANAKA Deparrmenr of Chemisuy. Tohyo Insrrrrtre oJ Tcclmolop?: Olrokqama. blegwo. Tolq o 152. Japarr

Reccrved 26 Apd 1982

The W cmissron of Cl, from a new vslencc-shell state havmg 0: symmetry (Te = 59774 cm-‘, re * 3.0 d) was ob- served by focunng =400 MI laser ndlatlon to gaseous chlormc. Evcltation us achlcvrd by vutuai t\\o-phoron absorption from rhe B ‘Q,: state formed by smglophoton absorption srep\rlscly. The cmlsslon spcc~n shoacd tranSluons IO the pound s13fe as uell as IO the repulsive geradc state dlssocurmg to CI(‘P) + CI(‘P) producrs.

In recent years, much attention has been paid to the higher lying electronic states of halogen molecules in connection with laser radiation in the vacuum ultra- violet and ultraviolet regions [ 1,2]. For molecular chlorine, a large number of emission and absorption

hnes have been studied [3]. However, little informa- tion is accessible about them except for the E 3floi state responsible for a discrete emission around

260 run (E + B). The purpose of tlti communication K to report the observation of a new electronic state by a three-photon transItion involving a sequential

step from the mound state (X ‘SIJ’) through the

I3 3fl,: state. When the radiation from a nitrogen laser pumped

dye laser (hlolectron W22/DL14) was focused into the quartz cell containing pure chlorine (Takacbiho Research grade >99.9%), ultraviolet emission could be observed at excitation wavelengths from 5 14 to

481 run (dissociation limit of the B state). Fig. 1 shows part of the excitation spectrum. Sohd lines show the excitation spectrum of the B 3flo;-X ‘Zg transition measured by the near-infrared emission, and dotted lines the W fluorescence excitation spec-

trum taken simultaneously. The W fluorescence was monitored by a photomultiplier (Hamamatsu TV R166) through a band-pass filter (Coming 7-54) in tie detection region of 240-290 run, and the signal was integrated with a boxcar(PAR 162/164). The strongest peak is located at 501.89 nm, where the P11 war&ion is overlapped by the R13 transition of the

- Vlslble _..._ “V

: t_ p11 R13

5015 502 0

Wavelength (nm)

502.5

rig. 1. Fluorescence cxalarion spectrum of Cl2 UI rhc vrrlblc and ultraviolel rcgrons. Cl, pressure ws 0.S Torr and the

laser bandwidth 0.4 cm-‘.

0 OOY-2614/82/0000-0000/S 02.75 0 1982 North-Holland 537

Volume 89. number 6 CHEMICAL PHYSKS LETTERS 9 July 1982

12-O band of 3sCl-35Cl, Though the UV fluorescence escltatlon peaks were observed irregularly, these hnes were clearly coincident with some of the visible ab- sorption lines (B .- X). Therefore, the escitation to tie upper state responsible for the UV emission origi- nates from the B state and the spectral intensities strongly depend on the dtiferenr resonance conditions

restricted by selection rules for the B + X transition and the transition to the higher stafe from the B state.

Fig. 2 shows the emission spectrum excited at 501.89 nm, which consists of two types of transition; (1) a Line shaped system below 250 nm and (2) a broad band system around 2 59 run. In this experiment, Cl? was irradiated in the pressure range of 0.1-3 Torr. No change in the spectral features could be observed. Assummg that the emission system (1) corresponded wth the transition to the ground state l , the relation

U(LJ”) = u&59774 cm-‘) - [G”(u) - G”(O)] 0)

was obtained by using molecular constants character- izing the ground state [5]. Here G”(u) denotes the vibrational term of ground state. ~00 corresponds to

three tunes the excitation frequency used, and it is

clear that three photons are involved in the excitation process. Since the intensities of predominant lines

l Opuul-opttnl double rcsonancc ckpetuncnn (to be pub- hjhcd) mdwxlc th3t IIUC~ uqerade &CIIO~IC stSCs C\ISI around 60000 cm-l above the pound st3tc Thcsc WHes arc reprcscntcd by rhc symbols Q, p, and 7 in order oi incre;lsmg cnerg) and the molecuku constm~& have been dcrncd. Anal)sis using these constitute shous that Ihc LIV cmwlon observed at 501.89 nm ortgilutcs from the -r state and that the cwkmon wasachtcvcd by the vetble ttansttton of B - S (12-O P, ,) folloacd by the vulu;rl two-photon

frmsmn (O-11 Q,J from k? U sfatc prl’donumntly.

Fp. 2. Emuston spectrum of Cl2 III the ultraviolet region cxc~rcd x 501.89 nm. Cl: pressure ws 3.0 Torr and the spcc- IKII bandwdth 0.2 nm.

around 235 nm decrease monotonically on either side of a single intensity maximum, it is then reasonable to

suppose that the fluorescence originates from the u’=O level. The fluorescence lifetime extrapolated to zero pressure is less than 30 ns and then vibrational relaxa-

tion could not have occurred under the present condi-

tions. The turning points of the potential curve in the

ground state were calculated by the RKR method [4]. Since the transition from the u’ = 0 level to the outer wall of ground state yields an intensity maximum, the

equdibrium nuclear distance of the upper state is roughly estimated to be 3.0 A T.

It should be noted here that the efficiency of cir- cularly polarized photons yielding the UV emission dropped by a factor of 5 at 501.89 MI as compared with that of linearly polarized ones. This polarization effect indicates that the virtual two-photon transition originates From the B state. According to the treat- ment by Bray and Hochstrasser [S], the ratio act/all

of simultaneous two-photon transition absorption cross sections for individual rotational lmes is 1.5 for alI branches of 352 = 0, C-l,52 transitions except for

the Q branch of the An = 0 transition. In the latter case the ratio may range from 0 to l/4 depending on the type of transition. Considering the symmetry of the B3Ilo: state, the upper state is then expected to be O’, (lx:, 3Y,, or 311U) in Hund’s coupling case c.

The electronic configuration of valence electrons in molecular chlorine may be expressed in the form,

(o,3P)‘(nu3P)4(ns3P)4(uu3P)o

and the electronic transitions may be classified as intravalence-shell or Rydberg. Valence-shell states arise from the excitation of one or two valence elec- trons to outer valence orbitals. It is then reasomble to consider qualitatively that this change in the val- ence-shell configuration makes the bond length much larger than that of the ground state. Though the ob- served transition is not fully characterized at this state, the excitation to a valence-shell state would be performed by the virtual two-photon transition from the B 311~ shte. According to poter.tial curves c&u-

t The turning pOInIs of the outer MU in the ground state uere calcuhtcd 10 be 3.02181 A for 17602.54 cm-’ above U” = 0,X99047 A for 17319.58 cm-‘,and 2.96051 A for 17026.67 cm-‘, whde the viinhotul energxs of the Bound state are 17572.68 cm-’ for U” = 41 and 17299.97 cm-’ for u” = 40.

528

Volume 89. number 6 CHCMICAL PHYSICS LEITERS 9 July 1981

lated by Peyerimhoff and Buenker [6], the vaence-

shell states of 311a and 311, lie at 057500 and 61000 cm -I, respectively, while Rydberg states even con- verging to the lowest ‘lip state of Cl; do not exist up to 67000 cm-l. The 311U state whicll dissociates to tile ionic products, C1”(3Ps) + Cl-(‘S& is consistent with our observed transition both energetically and symmetrically. And the estimated nuclear distance (z3.0 A) agrees with its theoretical value (z3.0 A). Unfortunately, they did not treat the 3Z, state which also converges to the ionic limit, Cl+(3P& + Cl’(‘Sg). Another electronic state bsiug syrnmetri- sally acceptable is 1X; lying at 65.500 cm-’ sligiltly

higher than our experimental vi-due. It should be noted here that the potential curves

calculated by Peyertioff and Buenker [6], indicate some repulsive electronic states witll gerade symmetry converging to Cl(‘P) + Cl(*P). The baud system around 259 MI is consistent with the E 311$ -t B 3n~; transition energetical.ly. However, the broad feature with a single intensity maximum at 259 nm strongly suggests that it can be assigned to the direct transition

from the upper ungerade state mentioned to UIC repul- sive gerade states, rather than the collision-induced for- mation of the E 3flo; state followed by the transitron (E- B).

The authors thank Dr. hl. Kawasaki at hli’c Univcr- sity for his interest arid valuable discussions.

References

[I] J.K. RICC, AK. Hays and J.R Wood\\orth. Appi. Phys.

Lelrcrs(1977) 31.

[3] AK. Hays. Opt. Commun. 28 (1977) 209. [ 31 K.P_ Hubcr and C. Hcnbcrg. Molecule rpectrs 2nd ndcc-

uhr strucuwz, Vol. 4. Constants of dnromic molecules (Van Nosrnnd, Prmccton, 1977) p_ 146.

[4[ A.E. Dou&sand A.R. Hoy, Can. J. Phys. 53 (1975) 1965.

[Sf R.G. Uri~y 2nd R.M. Hochrtnsscr. Mol. Phss. 31 (1976) 1199.

[6] SD. Peycrunhoffand R-l. Bucnhcr, Chrm. Phys. 57

(1981) 179.