R. P. Merrill

Vibrational relaxation during gas–surface collisions

The probability for deactivation of CO2(001) has been found to be 0.22 for collisions with a stainless steel surface, 0.16 for silver, and 0.20 for nickel. The probability for deactivation on the silver surface was invariant with surface temperature in the range from 298 to 473 K. A pulsed CO2 laser was used to excite the CO2, while the population of vibrationally excited molecules was monitored by observing their infrared fluorescence at 4.3 μm.

Angularly resolved temperature programmed decomposition—nitrogen emission from the decomposition of hydrazine on Ir(111)

Strong angular dependence has been found in the emission of N2 from the temperature programmed decomposition of hydrazine on oridium (111). (AIP)

Vibrational relaxation of carbon dioxide (101) and carbon monoxide (v = 2) during gas--surface collisions

The probability for deactivation of CO(v=2) and CO2(101) on collision with polycrystalline silver surfaces has been measured. The deactivation probability for CO(v=2) was found to decrease from 0.33 at 300 K to 0.20 at 440 K, while the deactivation probability for CO2 (101) was found to decrease from 0.72 at 300 K to 0.37 at 440 K. Since no population was observed in the CO(v=1) and CO2 (001) intermediate levels, it appears that each deactivation proceeds completely to produce the vibrational ground level. The magnitudes for the deactivation probabilities and the temperature dependencies indicate that a dominant mechanism for relaxation involves trapping and subsequent deactivation by one or more of several mechanisms, including electron–hole pair formation, vibration‐to‐rotation energy transfer, or perhaps even transfer of energy to the surface phonons. The experiments were performed in a UHV chamber by using a tunable infrared laser source to excite gas‐phase molecules vibrationally before their collisi...

Competition between direct‐inelastic and trapping desorption channels in the scattering of NO (v=0, J) from Ir(111)

The competition between direct‐inelastic and trapping‐desorption scattering of NO from IR(111) has been studied using multiphoton ionization and time‐of‐flight mass spectrometry. Molecules interacting by each mechanism were observed and characterized by their angular, velocity, and internal state distributions. For Ts 300 K only trapping‐desorption is observed. At Ts<300 K, the trapping fraction was ≂0.85. The shift in scattering mechanism appears to be caused by a change in the surface composition due to dissociation of chemisorbed NO near room temperature.

Direct‐inelastic and trapping‐desorption scattering of NO(v=0, J) from Ir(111): Angular, velocity, and rotational energy distributions

Angular, velocity and rotational energy distributions are reported for the scattering of NO from IR(111) at surface temperatures from 100–700 °K. (AIP)

Vibrational relaxation of carbon dioxide at LiF(100)

The vibrational relaxation of CO2 at LiF(100) has been investigated by monitoring infrared fluorescence from vibrationally excited molecules under conditions where they are relaxed primarily by collisions with the solid surface. The relaxation probabilities are found to be 0.65±0.10 at room temperature and 0.35±0.10 at 450 K. In order to understand better the vibrational relaxation results, angular distributions of CO2 scattered from LiF(100) were measured with a molecular beam scattering apparatus. At slow incident beam velocities, the trapping probability of CO2 at LiF(100) is essentially unity. Thus, in the vibrational relaxation measurements, where the incident velocity is even slower than in the scattering experiments, vibrational relaxation is preceded by trapping. Possible mechanisms for relaxation are discussed. Excitation of phonons in the solid and transfer of energy to other degrees of freedom of the molecule (i.e., translation, rotation, and other vibrational modes) are both plausible relaxati...

Laser Studies of Vibrational Energy Exchange in Gas-Solid Collisions

Recent measurements on the probability of vibrational deactivation in gas-solid collisions are summarized. The experiments were performed by using a tunable infrared laser source to excite gas-phase molecules vibrationally before their collision with the surface and by measuring the population of vibrationally excited molecules through their time-resolved infrared fluorescence. On polycrystalline silver surfaces, the deactivation probability for CO(v=2) was found to decrease from 0.33 at 300 K to 0.20 at 440 K, while the deactivation probability for CO2(001) was found to decrease from 0.72 at 300 K to 0.37 at 440 K. In contrast, the probability for dectivation of CO2(001) was found to be 0.16 and to vary only slightly with temperature. These observations are consistent with a mechanism involving trapping followed by electron-hole pair formation. In separate experiments, state-resolved scattering of rotationally cold NO from an Ir(111) surface held at 80K has demonstrated the exchange between translational and rotational energy. A similar technique employing vibrational excitation of the in-coming molecular beam should provide a very detailed picture of vibrational deactivation at surfaces.