Symmetry-breaking collisional energy transfer in the 4νCH rovibrational manifold of acetylene: spectroscopic evidence of a quasi-continuum of background states

Rovibrational Energy Transfer in the 4νCH Manifold of Acetylene, Viewed by IR−UV Double Resonance Spectroscopy. 1. Foundation Studies at Low J

“…The primary subbands in Figure 5 are 1 1 0 3 3 1 5 0 1 K 0 1 (i.e., K′ ) 1 r K ) l ) 0). Even-K′ features with K′ ) 0 and K′ ) 2 also occur; at higher J (>10, for example), these may arise from axis switching, 43,93,94 but at low J, they indicate key perturbations in the rovibrational state that mix in odd values of l ()K), notably l ) 1, 8,29,31 spoiling l as a "good" quantum number. Assignment is facilitated by reduced termvalue plots 95,96 to identify K′ and correlate P, Q, and R branches of the rotational structure.…”

“…This complements other optical double-resonance studies of collision-induced dynamics in C 2 H 2 (and its isotopomers) excited to fundamental and lower overtone/combination rovibrational levels (ω vib e 6500 cm -1 ). [40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] Much of our own work 7,8,[28][29][30][31]40,41,[43][44][45][47][48][49] has pursued the role of intramolecular perturbations in promoting (or, in some cases, inhibiting) the efficiency of rotationally selective vibrational energy transfer. [55][56][57] Numerous other acetylene high-overtone experiments entail techniques such as IR absorption, 6,[9][10][11][12][13][14][15] stimulated emission pumping (SEP), 4,58-60 dispersed LIF (both rovibronic 20,[23][24][25][26][27]61,…”

“…32 Our objective is to characterize and rationalize various unusual spectroscopic and dynamical effects observed and already concisely reported. 7,8 These effects include odd-∆J rotational energy transfer (RET) and J-resolved state-to-state vibrational (V-V) energy transfer. Especially novel and challenging are collision-induced quasi-continuous background (CIQCB) effects, underlying the discrete IR-UV DR features that are regularly observed.…”

“…This led us to postulate 31 that the ν CC + 3ν CH manifold is similarly susceptible to rovibrational perturbations by collision-induced Stark fields. Figure 2 displays some representative IR-UV DR results, 7,8 in which the UV PROBE wavelength is set to monitor the (1 0 3 0 0) 0 J ) 1 rovibrational level and the IR PUMP wavelength is scanned. This figure will be discussed progressively in the course of this paper.…”

Control of frequency chirp in nanosecond-pulsed laser spectroscopy 2 A long-pulse optical parametric oscillator for narrow optical bandwidth

“…Continuously tunable, narrowband coherent light with ns pulse duration is also required for various spectroscopic applications that entail a timeresolved molecular excitation sequence, such as our infrared-ultraviolet double-resonance studies of J-resolved, collision-induced rovibrational energy transfer in acetylene (C 2 H 2 ). [6][7][8][9][10] In a recent publication 11 we reported a major advance in the techniques that are required for control of the optical bandwidth and frequency chirp characteristics of output from a single-longitudinal-mode (SLM), continuously tunable OPO system based on the quasi-phasematched NLO medium, periodically poled KTiOPO 4 (PPKTP). This PPKTP OPO was injection seeded by a cw tunable diode laser (TDL) at a signal wavelength s of ϳ842 nm and pumped at 532 nm by a SLM Nd:YAG laser with a pulse duration of ϳ8 ns.…”