Doppler-Resolved Kinetics of Saturation Recovery

Abstract

Frequency-modulated laser transient absorption has been used to monitor the ground-state rotational energy-transfer rates of CN radicals in a double-resonance, depletion recovery experiment. When a pulsed laser is used to burn a hole in the equilibrium ground-state population of one rotational state without velocity selection, the population recovery rate is found to depend strongly on the Doppler detuning of a narrow-band probe laser. Similar effects should be apparent for any relaxation rate process that competes effectively with velocity randomization. Alternative methods of extracting thermal rate constants in the presence of these non-thermal conditions are evaluated. Total recovery rate constants, analogous to total removal rate constants in an experiment preparing a single initial rotational level, are in good agreement with quantum scattering calculations, but are slower than previously reported experiments and show qualitatively different rotational state dependence between Ar and He collision partners. Quasi-classical trajectory studies confirm that the differing rotational state dependence is primarily a kinematic effect.

Rotational energy transfer kinetics of optically centrifuged CO molecules investigated through transient IR spectroscopy and master equation simulations

“…Research on the collisional relaxation of rotational energy has a long history, and is motivated by efforts to understand the physical and chemical nature of high energy, non-equilibrium environments such as flames, plasmas, and gas-phase lasers, and of the products of exothermic reactions. 8–29 At 300 K, most molecules have energy gaps Δ E between rotational states that are significantly smaller than the thermal energy, and rotational energy transfer rates are large compared to rates for vibrational relaxation. Double-resonance and other techniques have been used to determine state-to-state rate constants for a number of systems and reveal common features about rotational energy transfer processes.…”

“…Double-resonance and other techniques have been used to determine state-to-state rate constants for a number of systems and reveal common features about rotational energy transfer processes. [11][12][13][14][15][16][17][18][19][20][21][22][23] The rate constants are largest when the change in angular momentum DJ is small, and decrease with increasing DJ. When the collision energy is reduced, the state-to-state rate constants have narrower distributions in DJ.…”

“…When the collision energy is reduced, the state-to-state rate constants have narrower distributions in DJ. [19][20][21][22][23] At 300 K, rotational energy transfer of thermally populated states can occur on nearly every gas-kinetic collision because of small energy gaps between rotational states. In some cases, rotational energy transfer is faster than the gaskinetic collision rate because of long-range attractive forces.…”

Contrast and Complexity in the Low-Temperature Kinetics of CN(v = 1) with O2 and NO: Simultaneous Kinetics and Ringdown in a Uniform Supersonic Flow

“…45−52 Photodissociation of BrCN at 248 nm is a good source of vibrationally excited CN(v = 1) although the nascent rotational distribution is highly excited, with a significant fraction at very high rotational levels. 49,50 The decreasing efficiency of rotational energy collisions for high angular momentum states, 53,54 where the rotational energy gaps exceed the typical collision energies, can result in an evolving bimodal rotational state distribution that includes a growing thermal component and a more persistent, highly excited component. 47,51,52 Our observations of a growing thermal component are consistent with these previous reports.…”

Inelastic Scattering of CN Radicals at the Gas–Liquid Interface Probed by Frequency-Modulated Absorption Spectroscopy

“…In this paper, we demonstrate the first application of frequency-modulated (FM) transient absorption spectroscopy to gas–liquid scattering. FM absorption spectroscopy has been widely used as a sensitive spectroscopic method, , which has been applied previously to probe gas-phase photodissociation dynamics, , reaction kinetics, , and collisional energy transfer dynamics. , The first allowed electronic transition of the cyanogen radical, CN­(A 2 Π-X 2 Σ + ), occurs in the near-infrared, convenient for many commercially available narrow-band continuous-wave laser sources. This makes it an excellent target species for transient FM spectroscopy.…”