Supersonically cooled deuterium chloride (10 K, DCl(J ≈ 0, 1)) has been scattered from the gas−liquid interface under thermal (E inc = 1.5(1) kcal/mol) and hyperthermal (E inc = 11.5(5) kcal/ mol) conditions for a series of prototypical liquids (perfluoropolyether (PFPE), squalane, and glycerol), with the final DCl quantum states detected with narrow-band IR laser absorption methods to achieve rotational (J) and transverse Doppler (v y ) resolutions. First of all, we see direct evidence for both trapping desorption (TD) and impulsive scattering (IS) components in the DCl distributions, with both TD rotational and transverse velocity components fully equilibrated with the liquid (T S ≈ T rot ≈ T Dopp ). Second, high-resolution laser dopplermetry on the IS component reveals quite efficient transfer from E inc into out-of-plane scattering of the DCl, in contrast with the notably inefficient channeling of the collision energy into end-over-end rotation. Third, though the rotational excitation efficiency is low, the impulsively scattered DCl(J) exhibits much hotter rotational distributions from gas−liquid interfaces dominated by polar (CF) versus nonpolar (CH) bonds (i.e., T rot(PFPE) ≫ T rot(squalene) ≈ T rot(glycerol) ). We interpret our results in terms of a simple kinematic "lever arm" collision model, by which the angle of the DCl striking the surface influences the torque delivered by changing the effective lever arm and thereby enhancing or diminishing rotational excitation. Finally, we extend this simple model to more realistic gas−liquid interfaces with surface roughness due to thermally activated capillary waves, which smooths out any sharp rotational structure. Of particular note is that such an averaging predicts a surprisingly Boltzmann-like distribution of final quantum states characteristic of a rotational "temperature", which is in agreement with many previous gas−liquid scattering studies with internal quantum state resolution.

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.

Molecular beams of supersonically cooled (T rot ≈ 10 K) carbon monoxide (CO) have been scattered from three lowvapor-pressure liquids (PFPE, squalane, and glycerol) over a range of surface temperatures (253−303 K), with the final rovibrational distributions probed by shot-noise-limited direct IR laser absorption methods. Specifically, the present work focuses on quantum-state-resolved scattering at low incident energies (E inc ≤ 1.0 kcal/mol), which would normally be expected to yield pure trapping desorption (TD) dynamics with CO in complete thermal equilibrium with the liquid (T rot ≈ T S ). By way of contrast, the nascently scattered CO(J) exhibits both rotational (T rot ) and Doppler translational (T Dopp ) distributions distinctly colder than T S , a phenomenon which is systematically reiterated over a wide range of liquid temperatures. To help identify the relevant collision physics responsible for this surprising subthermal behavior in CO, high-level ab initio potentials and detailed molecular dynamics simulations are explored for a series of projectiles (CO, DCl, and CO 2 ) with varying strengths of interaction with the liquid. At low incident energies, each of the more strongly interacting DCl and CO 2 projectiles is found to thermalize with the liquid interface (T rot ≈ T S ), while CO is predicted to emerge colder than the surface (T rot < T S ) and in remarkably quantitative agreement with experiment. Statistical analysis of the trajectories identifies that CO spends substantially less time and penetrates less deeply into the surface compared to DCl/CO 2 projectiles due to a combination of a shallow van der Waals well and a steep repulsive wall. The simulations reveal that low-energy CO does not undergo conventional trapping desorption (TD) at the gas−liquid interface but instead exhibits incomplete warming from its jet-cooled value (T rot ≈ 10 K) via an unexpected subthermal impulsive scattering (STIS) pathway. The data suggest that non-equilibrium IS dynamics at low energies may play a crucial role in inelastic energy transfer and thermal accommodation at the gas−liquid interface for weakly interacting collision systems.