State-Resolved Studies of OCS Scattering at the Gas–Liquid Interface: Tests of Landau—Teller/Rapp Theory for Rotational vs Vibrational Energy Transfer

Abstract

Quantum-state-resolved collisional energy transfer of jet-cooled carbonyl sulfide (OCS) at the gas−liquid interface (E inc = 2.2(4) kcal/mol) has been explored with a powerful combination of molecular beam scattering and highresolution direct absorption IR spectroscopy, which has permitted the first characterization of rotational, vibrational, and transverse Doppler excitation/ accommodation dynamics on a liquid for a polyatomic projectile. The results are consistent with the complete rotational and transverse translational equilibration of the incident OCS (T rot,trans ≈ 10 K) with the surface for a variety of liquids (perfluorinated polyether (PFPE), squalene, and glycerol) and liquid temperatures (T S = 263−303 K). In dramatic contrast, however, vibrational populations in the ground (00 0 0), v 2 OCS bend (01 1 0), and ν 1 CS stretch (10 0 0) modes for the scattered OCS species remain far out of equilibrium with the liquid surface, indeed exhibiting adiabatic, spectator-like behavior and remaining identical (within experimental uncertainty) to vibrational temperatures observed in the incident OCS molecular beam. This spectator-like behavior for vibrational scattering clearly suggests that polyatomic vibrational coordinates must be treated separately from the other degrees of freedom, at least for low energies explored herein characteristic of the thermal desorption TD pathway. Such vibrationally decoupled behavior at energies and repulsive wall potentials sampled by the TD channel is shown to be in excellent qualitative agreement with expectations from high-level ab initio calculations and well-known Landau−Teller/Rapp vibrational energy-transfer models.

Quantum-State-Resolved Scattering of OCS at the Gas–Liquid Interface: Hyperthermal versus Thermal Vibrational Equilibration Dynamics in Polyatomics

“…We again stress that the crucial comparison in Figure 5 is the contrasting behavior between the linearly increasing solid blue line for T vib (TD′) and the reddashed line with vanishing slope for T vib (IS). At first glance, this result seems counterintuitive, with vibrational excitation for the TD′ channel at high energies behaving similarly to what might have 37 been expected for TD scattering at low energies, i.e., complete equilibration with the liquid T S for all vibrational/rotational/translational degrees of freedom. By way of comparison, the blue-dashed line in Figure 5 represents the limit of full vibrational equilibration (T vib ≈ T S ), against which the T vib (TD) data (blue points) and linear fit (blue solid line) clearly demonstrate significant vibrational warming from the incident beam (T vib (inc) = 220 K).…”

“…The gas−liquid scattering laser spectrometer apparatus used in this work for quantum-state-resolved scattering has been described elsewhere 16,28,37,38 and warrants only a brief…”

“…3,19,40 With balanced, fast analog electronics, subtraction of signal and reference beams routinely achieves nearly shot-noise-limited absorbance sensitivities of ∼1 × 10 −5 in a 10 kHz experimental bandwidth for ∼20 μW laser power on each detector. 16,37 The scattering measurement system consists of a piezoelectric molecular beam source, a vertical rotating wheel liquid target, and a laser multipass, with the laser propagating perpendicular to the scattering plane formed by the pulse molecular beam axis and the rotating wheel surface normal. The piezoelectric beam source 41 contains 5% OCS in a balance of H 2 expanded through a 500 μm orifice at a stagnation pressure of ∼70 Torr, conditions which optimize signal while still ensuring operation in a OCS nonclustering regime with the signal linearly proportional to the backing pressure.…”

“…The dual slope behavior of the logarithmic data in Figure 3 reflect rotational distributions clearly characterized by a two-temperature Boltzmann model, as often observed in hyperthermal scattering experiments. 16,37,44,45 These population fits assume a common Gaussian distribution in the lower-state velocity components, whereas a more flexible and accurate description allows for different Doppler distributions for the TD and IS channels. This model achieves excellent least-squares Doppler fits to the high-resolution spectral scans (e.g., Figure 1), with velocity distributions depending on the TD/IS branching ratios characteristic of each vibrational state manifold.…”

“…The scattering spectral data can be fit to eq 3 by floating six temperatures (T rot/trans/vib for both the TD′ and IS channels), the branching ratio (α), and the overall normalization (N). To reduce parameter correlation, the TD′ rotational and transla-tional temperatures are fixed at the bulk liquid values (T rot (TD′) ≈ T trans (TD′) ≈ T S ), i.e., a rotational/translational equilibrium condition explicitly demonstrated in the previous study, 37 though such a constraint does not change any reported rovibrational temperatures within experimental uncertainties. A sample spectral fit region is illustrated in Figure 1, where the sequential blowup highlights both the dualtemperature Doppler line shapes (e.g., broadening of the Gaussian base) and dramatic differences in signal strengths for transitions arising from (00 0 0), (01 1 0), and 00 0 1) lower-state vibrational manifolds.…”