Electronic spectroscopy, lifetimes and barrier to linearity in the A 1B1–X 1A1 system of CBr2 CHONG TAO, D. BRUSSE, Y. MISHCHENKO, C. MUKARAKATE and S. A. REID, Department of Chemistry, Marquette University, Milwaukee, WI 53233. 62nd International Symposium on Molecular Spectroscopy, June 18th, 2007

Background on CBr2 The spectroscopy AX system of CBr2 was first detected in Ar matrix by Andrews in 1968.a In 1990, Zhou et al. reported the first room-temperature gas phase spectrum in their cross-beam apparatus and assigned the origin at 14885 cm-1, but the bandwidth(~40cm-1) limited spectra information and precision.b In 1993, Xu and Harmony(XH) carried out the most extensive study of CBr2, recording the jet-cooled LIF spectrum of CBr2 through pyrolysis of CHBr3, and reassigned the electronic origin to ~15092 cm-1.c Chang and co-workers recorded the emission spectra of CBr2 in a discharge source. However, the CHBr and Br2 generated in the beam greatly interfered with the CBr2 spectrum.d From our experience of halocarbenes, we employed an improved recipe to generate CBr2 without the interference of CHBr and Br2, by pulsed discharge of a gas mixture of CHBr3/CBr4 seeded in Ar. a: JCP., 48, 979(1968) b: CPL., 166, 547(1990) c: JPC., 97, 7465(1993) d: PCCP., 7, 2468(2005)

CBr2: Xu and Harmony 7466 The Journal of Physical Chemistry, Vol. 97,No.29,1993 Figure 1. Prominent progressions of A  X transition of CBr2

Fluorescence excitation survey spectrum of CBr2

Fluorescence excitation spectrum and simulation Individual isotopomer is simulated and C79Br2, C79Br81Br, C81Br2 are scaled to a 1:2:1 ratio For each band, we obtained the band origins and A rotational constants for all 3 isotopomers.

Revision in origin assignment based on excited state isotope shifts The isotope shifts can be fit to the expression: We expect that DT00 is very close to zero CBrCl(-0.08cm-1), CCl2 (-0.22cm-1) This is true if we shift the origin position of Xu and Harmony by one quantum, to~15,279 cm-1 DT00 =-0.02cm-1

Vibrational parameters of à state   T00    Reference Experiment C81Br2 15278.78(48) 472.57(46) 183.11(11) -0.539(44) -0.021(6) This work 15092.7(16) 473.8(12) 183.6(4) -0.62(13) -0.038(20) XH C79Br81Br 15278.86(38) 472.99(35) 184.13(8) -0.519(35) -0.024(4) 15091.5(5) 475.0(5) 184.9(1) -0.71(7) -0.058(9) C79Br2 15278.67(46) 473.58(44) 185.19(11) -0.537(42) -0.026(5) 15092.5(17) 474.8(12) 185.4(4) -0.040(21) Theory MRCI 15237 a CASSCF 16486 444.6 177.9 CASPT2 14191 484.0 194.6 15192 b XH: JPC, Vol. 97,No.29, 7466, 1993 a: JCP, Vol.112, 2227, 2000 b: JPC(A), Vol.106, 1357, 2002

Dependence of (A-B) constant on bending quanta A linear trend is observed for the A rotational constants vs. bending quantum number These can be fit to the expression: The deviation from linearity at high energy reflects the onset to linearity ~18800 cm-1, consistent with theoretical prediction ~ 19065 cm-1.* * JCP, Vol.112, 2227, 2000

Variation of fluorescence lifetime with energy The lifetimes show little dependence on Ka over the measured range The measured lifetimes are consistent with previous matrix and gas-phase studies Bondebey and English:* Ar matrix measurement:14.5s Lee**:Gas-phase at~18000 cm-1:3.4 s *:JMS. 79, 416(1980) **:PCCP. 5, 3859(2003)

SVL emission spectra The only experimental study about the vibrational structure was carried out by Chang and co-workers by exciting (0,12,0) and (0,13,0) bands of CBr2. Surprisingly, they failed to observe the pure bending progression in the emission spectra.* They suggested that the congested structure beginning ~3650 cm-1 about the X 1A1 origin was transitions to a 3B1 state, which is also problematic, since: 1. This is ~ 2200 cm-1 smaller than theoretical prediction of ~5900cm-1.# 2. Dunham expansion fit to the observed levels gives a small standard deviation ~2.3 cm-1, suggesting a very small spin-orbit interaction. *: PCCP., 7, 2468(2005) #: JPCA., 106, 8495(2002)

SVL emission spectra Pumping K`a = 0 of various bands of C79Br81Br, we probed level up to 7000 cm-1 above X state.

SVL emission spectra Polyad structure due to the near 3:1 resonance of the ground state symmetric stretching and bending frequencies. Since only 1-2 levels in each polyad display significant FC activity, the spectrum retains a regular appearance.

Ground state vibrational constants Parameter Dunham fit 1 606.6(4) 2 199.5(2) 3 679.8(7) 11 -2.01(4) 12 -1.23(2) 13 -10.66(29) 22 -1.49(1) 23 -2.60(10) 33 Fixed at 0.0 ab initio* 601.6 196.6 655.3 Parameters of C79Br81Br Anharmonic frequency 3 Fit 92 levels with =2.7 cm-1 *JPCA., 103, 7900(1999)

Summary Using an improved recipe, LIF and SVL emission spectroscopy of A 1B1–X 1A1 system of CBr2 were studied. The electronic transition origin was reassigned to 15279 cm-1, from isotope splitting. Vibrational constants for both ground(X) and excited(A) states were obtained. Fluorescence decay lifetime decrease dramatically with excitation energy and show no obvious dependence on K`a quantum number. However, a sudden increase of A rotational constant near 18800 cm-1 indicates the approach of linearity. Small standard deviation for the fit of the X state vibrational levels up to 6000 cm-1 indicates very weak/ no spin-orbit interaction between the X and low-lying triplet state, which is predicted ~5900cm-1.* *:JPCA., 106, 8495(2002)

Acknowledgements People: Calvin Mukrakate Mihaela Deselnicu Yulia Mishchenko Daniel Brusse Robert Forner Dr. Scott Reid Dr. Timothy Schmidt Dr. Scott Kable Funding: $ National Science Foundation