1. 1 B. RAM PRASAD, MANGALA SUNDER KRISHNAN Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India. AND E. ARUNAN Department of Inorganic and Physical Chemistry Indian Institute of Science, Bangalore 560 012, India. PERTURBATION TREATMENT OF STARK EFFECT ON TORSIONAL ENERGY LEVELS

2. 2 Acknowledgements  Director IIT Madras,  Dr. S. Karthikeyan,  Dr. Pankaj K Mandal (IISc Bangalore), Funds:  IIT Madras,  IITM Alumni Association,  Department of Science and Technology India (DST),  Tamilnadu State Council for Science and Technology (TNSCST)  Conference Organizers.

3. 3 Spectral assignment of benzene – water dimer Stick diagram of the J = 3 → 4 progression observed for the C 6 H 6 - H 2 O dimer. A. Assignment given by Gutowsky et al. for the m=0 and m=1 states. A B B. Assignment given by Emilsson et al. based on the Stark measurements. Dipole moments obtained by Emilsson through fitting: m=0 ----- 1.65 Debye and m=1 --- 2.00 Debye. T. Emilsson, H. S. Gutowsky, G. de Oliveira and C. E. Dykstra, J. Chem. Phys., 112, 1287 (2000). H. S. Gutowsky, T. Emilsson, and E. Arunan, J. Chem. Phys., 99, 4883 (1993).

4. 4 Original and revised Assignments Is this progression really arising from three different states, attributed to the three J = 1 states of H 2 O ? or any error in the interpretation of Stark splitting?  Original could be fitted to a VRT Hamiltonian, with most of the lines assigned to two progressions, that of m = 0 and m = 1 all in blue Stick diagram of the J = 3 → 4 progression observed for the C 6 H 6 - H 2 O dimer.  Revised assignment based on Stark effect, divides the m=1 progression into three different states, in blue, red and black.

5. 5 13 CC 5 H 6 -H 2 O and C 6 H 5 D-H 2 O spectra  Rotational spectra of these asymmetric tops showed similar m=0 and m=1 progressions.  Both series were split due to asymmetry.  The spectra of the three isotopomers (including parent C 6 H 6 -H 2 O ) could be fit to the same VRT Hamiltonian. B. Ram Prasad, Mangala Sunder Krishnan and E. Arunan, J. Molec. Spectrosc., 232, 308 (2005).

6. 6 Expressions used by Emilsson et al. for the calculation of dipole moments: The second order expression: These expressions are standard symmetric top semi rigid rotor Stark shift expressions. They do not contain explicitly the effect of the torsional degree of freedom. The first order expression:

7. 7 VRT Hamiltonian in the presence of an electric field can be written as, and where, Stark energy corrections with the effect of torsion: Yun-Bo Duan, Hui-Min Zhang, and Kojiro Takagi, J. Chem. Phys. 104, 3914 (1996).

8. 8 Assuming the internal rotation is periodic: the dipole moment operator is, The transformed dipole moment operator is, Yun-Bo Duan, and Kojiro Takagi, J. Chem. Phys. 104, 7395 (1996).

9. 9 The effective dipole moment operator for the ground vibrational state is, Therefore, Stark Hamiltonian in our calculations: Calculating all the required matrix elements like,

10. 10 First order Stark correction: Second order Stark correction:

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13. 13 Numerical test case: H. S. Gutowsky, T. Emilsson, and E. Arunan, J. Chem. Phys., 99, 4883 (1993).

14. 14 Future directions: The dipole moments and the assignments given by Emilsson et.al., have to be verified with the torsional dependent RSPT Stark expressions given here. Thank you