65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 1 TIME-RESOLVED ROTATIONAL ENERGY TRANSFER AND SPECTRAL LINE BROADENING IN ACETYLENE. A HIGH RESOLUTION RAMAN STUDY. J. L. DOMENECH, R. Z. MARTINEZ AND D. BERMEJO Laser Spectroscopy Group, Molecular Physics Department Instituto de Estructura de la Materia (CSIC) Serrano 123, Madrid, Spain

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 2 MOTIVATION Rotational energy transfer rates are relevant in a number of important problems: energy balance in astrophysical environments, non LTE problems, laser dynamics, formation of spectral line shapes, etc. (Dynamicist’s point of view) High accuracy PES’s combined some calculation (CC, CS, classical trajectories…) allow the calculation of both the state-to-state rotational energy transfer cross-sections and the pressure broadening coefficients in ample ranges of pressure and temperature. (Theoretician’s point of view) The knowledge of pressure broadening and line mixing coefficients is essential for the accurate retrieval of concentrations, temperature distributions, etc. in remote sensing applications (earth’s atmosphere, combustion diagnostics, astrophysical media). It provides insight into molecular collision dynamics and allows for a very stringent test of intermolecular PES’s. (Spectroscopist’s point of view)

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 3 Introduction Pressure broadening and line-mixing of vibration-rotation lines are largely dominated by rotationally inelastic collisions. The collisional width of isotropic Raman Q-branch lines is not affected by reorienting elastic collisions or pure dephasing collisions, only by inelastic processes. If vibrational relaxation and vibrational dephasing processes are slow In principle, by an inversion procedure, the k’s can be obtained from the widths as a function of J. However, the number of unknowns is much larger than the experimental data available, and it is necessary to resort to fitting and scaling laws that relate the rate coefficients to a smaller number of parameters (EGL, IOS, ECS,…) Isotropic Raman (polarized part of the Q-branches) Dipole transitions and anisotropic Raman (O and S branches)

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 4 Introduction A more direct approach to obtain the state-to-state rates is to monitor the time evolution of non-equilibrium populations with rotational state resolution. Time-resolved pump-probe techniques have been widely used to address this type of problems: –Pump: IR absorption, Stimulated Raman effect, etc. –Probe: LIF, REMPI, IR absorption, etc. If there are no other gain (pumping is over), or loss mechanisms (vibrational relaxation, fluorescence, diffusion out of the probe volume), the change of population of a given level is due to rotationally inelastic collisions, and can be expressed:

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 5 MOTIVATION Our group has some experience in the measurement of collisional effects on high resolution Raman and IR spectra. We have also developed a Raman-Raman double resonance technique for the study of vibrationaly excited states, that we have improved with time resolution capabilities and J selectivity. We have the tools to obtain both the state-to-state RET rate coefficients and the Raman pressure broadening coefficients with the same experimental setup. Can our experiments provide some information on: the interplay of elastic/inelastic processes, vibrational contributions to the line broadening, independence of the RET rates on the vibrational state, etc.?

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 6 “Inverse Raman Spectroscopy”, aka Stimulated Raman Loss Spectroscopy It is a particular case of Coherent Raman Spectroscopy (CARS, SRS, RIKES, SRGS, ASTERISK, …) Four EM fields couple through  (3), that becomes resonant when the frequency difference between any two of them matches a Raman allowed transition.  1  2  3  4 RR RR  1  2 Inverse Raman Non-degenerate Four wave mixing

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 7 Raman-Raman double resonance with time resolution Ground state J J J 591 nm Bombeo Espectroscopía V=2 V=1 532 nm Spectroscopy Ground state J J J V 2 =2 V 2 =1 Virtual state Virtual state 1 0 Pump tens of ns Both processes use the stimulated Raman effect. If we pump a single level of v 2 =1 through a Q(J) transition of the fundamental band, and record the intensity of Q(J) transitions of the hot band (v 2 =2←v 2 =1) at controlled time delays, (i.e. number of collisions) we can monitor the time evolution of the rotational populations, and obtain the state-to-state coefficients.

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 8 Experimental Setup

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 9 Rate constants: Experimental approach For ortho-acetylene at 155 K, we can observe up to J=21. Considering only the odd levels, there are 55 unknowns in the k matrix (detailed balance relates excitation and de-excitation rates). Our approach is to pump each odd-J level in v 2 =1 and and for each one of them record the population of all accessible levels as a function of the time delay between pump and probe stages. All the data are fitted to the integrated form of the master equation: Eigenvectors of k Eigenvalues of k

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 10

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 11 Rate constants: Experimental results 155 K

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 12

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 13 Experimental results: the k-matrix Units:  s -1 mbar -1 ;

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 14 Rate constants: summary We have developed a time-resolved pump probe technique for the measurement of state-to-state rotational energy transfer rate constants that uses the stimulated Raman effect both in the pump and probe stages. Advantages: simple linear relationship between signal and populations. Our numerical approach accounts for multiple collision events. Disadvantages: experimentally complex, limited sensitivity. It has allowed the determination of the state-to-state rotational energy transfer coefficients for ortho-acetylene at 155 K WITHOUT resorting to any fitting or scaling law (for J<17). We have nevertheless checked how well some of these laws reproduce our results: EG, PEG, IOS-P, ECS. Doménech, Martínez, Bermejo, JCP 132, (2010)

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 15 We have recorded the Q-branch of 2 at 165 K and 298 K Pressure range:0.5 – K 0.5 – K All the Q-branch is recorded in a single scan ~3 cm -1, step cm -1 All lines within a scan are simultaneously fitted –The 0.5 mbar is fitted to a sum of Gaussians with the same width –γ Doppler = K; γ Doppler = K; –γ ILS = cm -1 –The rest of traces are fitted to a sum of Voigt profiles with γ G fixed to the 0.5 mbar value Lorentzian widths, for each J, are fitted to Linewidths: Experimental approach

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 16 Linewidths: results

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 17 Linewidths: results

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 18 Linewidths: results

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 19 Linewidths: room T

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 20 Linewidths: K

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 21 Summary and future work Work is in progress. Excess noise and chirping in the PDA have to be solved. Our time-resolved Raman-Raman double resonance set-up can provide state-to-state rotational energy transfer rate constants without resorting to fitting laws. The (provisional) broadening coefficients are compatible with the total depopulation rates al low T and with previous measurements at room T. Multi-spectrum fits with Galatry or speed-dependent Voigt profiles. Pure rotational Raman spectra. Line-mixing predictions. C 2 H 2 PES (Bartolomei et al. IFF CSIC) Extension to other systems of atmospheric, astrophysical or combustion relevance: N 2 -N 2 ; O 2 -O 2 ; x-H 2.

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 22 Linewidths: results

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 23

65th OSU International Symposium on Molecular Spectroscopy June 21-25, 2010 José Luis Doménech Instituto de Estructura de la Materia 24