Stimulated emission pumping spectroscopy via two-color resonant four-wave mixing

Abstract

A model calculation for nonimpact four wave mixing AIP Conf. Proc. 216, 323 (1990); 10.1063/1.39930Field and pressure induced fourwave mixing line shapes AIP Conf. Proc. 172, 249 (1988); 10.1063/1.37365Fourwave mixing spectroscopy of state selective collisions in gases and solids AIP Conf.We present a combined theoretical and experimental study of the application of two-color resonant four-wave mixing ͑RFWM͒ to stimulated emission pumping ͑SEP͒ spectroscopy. The theoretical approach employs time-independent, diagrammatic perturbation theory and a spherical tensor analysis in an extension of a recent treatment of degenerate four-wave mixing ͓Williams, Zare, and Rahn, J. Chem. Phys. 101, 1072 ͑1994͔͒. The resulting signal expression for two-color RFWM separates the molecular properties from purely laboratory-frame factors determined by the polarizations of the input beams and the rotational branch types of the SEP PUMP and DUMP transitions. This expression is valid in the limit of weak fields and for molecules in which the total angular momentum ͑omitting nuclear spin͒ is a good quantum number. In addition, we demonstrate that the spectral response for tuning the DUMP laser is a simple Lorentzian in free-jet experiments. We test our theoretical results and demonstrate the applicability of RFWM-SEP to jet-cooled, transient species in experiments on C 3 and HCO. Using the well-studied à 1 ⌸ u -X 1 ⌺ g ϩ system of C 3 , we illustrate and compare the two possible schemes for RFWM-SEP. These are defined as 1 ϭ 2 ͑PUMP͒ and 3 ϭ 4 ͑DUMP͒ or 1 ϭ 4 ͑PUMP͒ and 2 ϭ 3 ͑DUMP͒, where 1 , 2 , and 3 are the input frequencies and 4 is the signal frequency. Using the B 2 AЈ -X 2 AЈ system of HCO, we obtain RFWM-SEP spectra that probe ground-state vibrational resonances lying above the low threshold for dissociation to HϩCO. Varying the polarization of the input beams or PUMP rotational branch produce dramatic effects in the relative intensities of rotational lines in the RFWM-SEP spectra of HCO; these effects are well-described by our theoretical analysis. Finally, RFWM-SEP spectra of HCO resonances that are homogeneously broadened by dissociation are consistent with the theoretically predicted Lorentzian line shape; the full widths for these levels are in good agreement with those determined via unsaturated fluorescence depletion SEP.

The effect of laser bandwidth on the signal detected in two-color, resonant four-wave mixing spectroscopy

“…18 Evaluating the limits for the Gaussian probe laser line shape is more complicated. Indeed, while linear techniques would give rise to a signal profile described by a Voigt profile thereby allowing for the extraction of the transition width, 40 no analytic method is available to deconvolve the first folding integral in Eq.…”

“…These laser sources exhibit a relatively broad line shape determined by numerous factors such as pulse-to-pulse variations in the pumping-laser intensity, thermooptical effects on the dye cell exposed to the pumping light, cavity longitudinal mode competition, mechanical instabilities of the cavity, and ͑when used͒ nonlinear frequency doubling of the final laser output in birefringent crystals. These phenomena cause frequency chirp and stochastic fluctuations of the phase and amplitude of the optical field resulting in typical bandwidths of 0.1-0.01 cm Ϫ1 , [8][9][10][11][12][13][14][15][16][18][19][20] a value comparable to molecular transition widths in many situations of interest. Also, depending on the characteristics of the pumping laser and the active medium as well as on the optical resonator design, the dye-laser line shape may either appear as a sharp peak centered at the selected frequency or, in the opposite extreme, show a significant, irregularly structured background produced by amplified fluorescence.…”

“…8 These attributes have led to successful application of TC-RFWM to the study of numerous stable and transient species. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Diagrammatic perturbation theory ͑DPT͒ has proven to be a very effective tool in analyzing the spectra obtained in gas-phase TC-RFWM. Analytic expressions for the signal line profile have been reported for a variety of TC-RFWM schemes used in double-resonance spectroscopy of bound 23 and quasibound 24 molecular states.…”

Excited-state Structure and Dynamics of Nitric Oxide probed by Resonant Four-wave Mixing Techniques

“…Toward this end, a description of two-colour LIGS based on an extension of work on DFWM (Williams et al 1994b) that combines timeindependent diagrammatic perturbation theory with spherical tensor analysis has very recently been developed and applied to measurements of intensities and line shapes in two-colour RFWM experiments on C3 and HCO (Williams et al 1995). These methods look very promising for quantitative interpretation of line intensities observed by using typical experimental arrangements and a variety of polarisation configurations.…”

“…For example, DFWM has been used to perform background-free transition detection in stimulated-emission spectroscopy (Zhang et al 1992;Vaccaro 1994). Two-colour LIGS has been demonstrated in free jets (Butenhoff and Rohlfing 1992) and used in double-resonance spectroscopy of many stable (Buntine et al 1992(Buntine et al , 1995McCormack et al 1993McCormack et al , 1994McCormack et al , 1995Butenhoff and Rohlfing 1993a;Dunlop and Rohlfing 1994;Wheeler et al 1993) and transient species (Gray et al 1993;Williams et al 1994aWilliams et al , 1995Butenhoff and Rohlfing 1992). In addition to spectroscopy, the time evolution of the transient gratings produced in LIGS has been used as a powerful method to probe inter-and intramolecular dynamics (Rose et al 1987).…”