The states of the C 3 -Ar and C 3 -Kr van der Waals Complexes: Fluorescence Polarization and Saturation Jun-Mei Chao, Kan-Sen Chen, Shin-Shin Cheng, Anthony J. Merer, and Yen-Chu Hsu IAMS, Academia Sinica P. O. Box , Taipei, Taiwan, R.O.C. Supported by Academia Sinica, Taiwan, and National Science Council, Taiwan, R. O. C.

Introduction The spectrum of the C 3 1 Π u – 1 Σ g + system was first observed in comets by Huggins in Its first laboratory study was reported in 1942 by Herzberg and his co-workers. Since then many studies of the comet system of C 3 have been carried out. low bending frequency (  63 cm -1 ) of the state Renner-Teller effect of the state (  =0.35) ─ 1965, Gausset et al., vibrational and rotational analysis of the state. ─ 1994, W. J. Balfour et al., more vibronic bands were reported. ─ 2003, B. J. McCall et al., reassignment of the R(0) line of the ( ) band. ─ 2005, G. Zhang et al., perturbations of the,000 state have been observed and analyzed. We have utilized the comet system to study the C 3 -Rg (Rg=Ne, Ar, Kr and Xe) van der Waals complexes (G. Zhang et al., J. Chem. Phys. 120, 3189(2004)). The states of these complexes are not well understood; the effect of the rare gas atom on the Renner-Teller effet of C 3 has not been found.

A'A' A"A"    unique level   unique level  V b =2 V b =1 V b =0 Ã 1  u 

Top PMT Side PMT Polarizer Rhomb Allene + Rare gas ArF Laser LIF Laser beam X Y Z F⊥F⊥ S top =  top (F ⊥ + F ⊥ ) F⊥F⊥ S side =  side (F // + F ⊥ ) Extinction ratio = 1x10 -3 Polarizer F // F⊥F⊥

Top PMT Side PMT (perpendicular band) (parallel band) ( 1 ’, 2 ’, 3 ’- 1 ”, 2 ”, 3 ”) Upper Renner-Teller Component The spectrum of C 3 ( The spectrum of C 3 (excitation laser is horizontally polarized) (F // + F ⊥ ) (F ⊥ + F ⊥ )

C 3 Ar (perpendicular band) (parallel band) C 3 Kr C 3 Xe C 3 Ne (F // + F ⊥ ) (F ⊥ + F ⊥ ) (F // + F ⊥ ) (F ⊥ + F ⊥ )

Conventional polarization spectroscopy was obtained by modulating the fluorescence intensity by rotating the polarizer, which placed in front of the detector. And the Polarization Ratio (conventional) = In this work, we simultaneously detect the fluorescence signals. The advantage is that shot-to-shot intensity fluctuation from the lasers can be minimized. The polarization ratio defined in this work is, Polarization Ratio =

References, 1. R.N. Zare, Angular Momentum. Understanding Spatial Aspects in Chemistry and Physics. (Wiley, New York 1988). 2. J.T. Hougen, The Calculation of Rotational Energy Levels and Rotational Line Intensities in Diatomic Molecules. (NIST, Gaithersburg, 2001).

Table I. Calculated Polarized LIF Intensities

Polarization Ratio of the R(0) line of C band, calculated value from Table I: (F // + F ⊥ ) exp. (F ⊥ + F ⊥ ) exp. 2.6 ± 0.2 = 2.6 ± 0.2 (F // + F ⊥ ) cal. (F ⊥ + F ⊥ ) cal 4.5 = 4.5 Collisional Depolarization or Signal Saturation?

n=0.45  0.02 We did the experiments here!! Laser Power Dependence S = a × I n log S = log a + n log I n=0.82  0.07

2000 torr, 5% allene in He gas mixture expanded through a 500  nozzle R P Dye laser pulse energy 0.39mJ R 02 2 Q (F // + F ⊥ ) (F ⊥ + F ⊥ )

b 12 ρ b 21 ρA 21 A

           

Results and Discussion Difficulty of this experiment: due to our way of generating C 3 molecules by p h otolyzing allene, it is difficult to keep initial C 3 concentration constant. T h e transition probability of band is about 1.4 times of band. Qualitative understanding is possible at lower laser power. Further simulation is necessary.

Future Work Complete the polarization measurements of the C 3 bands especially the high power regime. Apply fluorescence polarization study to characterize the C 3 -Rg bands.