CO on NaCl(100): Model system for investigating vibrational energy flow

Abstract

Vibrational energy flow from CO physisorbed on NaCl(100) has been investigated. Infrared spectroscopy of monolayer CO reveals the narrowest linewidth reported for any surface-bound molecule. The vibrational band at 2154.98 cm -I has a full-width at half-maximum (FWHM) of 0.09 cm -1 at 4 K. We believe that this residuallinewidth is due to imperfections of the crystal face. Nevertheless it places a lower limit for an vibrational relaxation processes of 60 ps. The linewidth increases above 10 K, a behavior which can be accounted for by a vibrational phase relaxation mechanism. Attempts to photodesorb CO from its substrate by laser excitation into the vibrational absorption band reveal an exceedingly inefficient quantum yield of <1>,,;; 10 ... 7 • Finally laser excitations of a multilayer of CO on N aCl ( 100) at 22 K demonstrate that vibrational energy can be stored in the adsorbed CO long enough to allow fluorescence however relaxation to the substrate reduces the fluorescence lifetime from that of the bulk crystal. These measurements (bandwidth, vibrational desorption quantum yield, and fluorescence lifetime) of monolayer and multilayer CO on NaCl( 100) indicate that the vibrational motions are poorly coupled to this alkali halide substrate.

Theory of vibrational equilibria and pooling at solid-diatom interfaces

“…In all cases, there is then superimposed on these rapid exchanges the slow decay of N. The theory may be extended to treat isolated molecules in matrix solids, 9 and multilayers of CO on NaCl(100). 4,5 The validity of the equilibration approximation depends on the absorbed laser intensity. It may be expected to be valid when the intensity is sufficiently high.…”

“…1,2 In this paper, we describe such a statistical theory to explain two key effects: [1][2][3][4][5] (1) an inversion of the population of CO vibrational states and (2) the origin of the single exponential overtone fluorescence decay, the many contributing second-order and first-order steps in the mechanism notwithstanding.…”

On the infrared fluorescence of monolayer 13CO:NaCl(100)

“…5 All simulations are at 22 K, the temperature of the experiments. [3][4][5] We define (dP n /dt) pd as the net rate of pooling and depooling of state n with nearest neighbors. The pooling and depooling terms are described in Eqs.…”

“…We study the 13 CO monolayer rather than a 12 CO system following the experimental work of Ewing et al [3][4][5] and the simulation of Corcelli and Tully. 6,7 This isotope was chosen for the experiments to enhance the overlap of its fundamental frequency with the 12 CO gas laser emission.…”

“…[1][2][3][4][5] Several low temperature experiments have been conducted on monolayers and multilayers of 12 CO and 13 CO, observing overtone (n + 2 → n) infrared fluorescence from monolayer 13 CO excitation between the 2nd and 16th vibrational state 5 and multilayer 13 CO excitation reaching the 30th vibrational state. 3,4 In the monolayer case, these states were inferred by application of a filter (4200-3400 cm −1 transmitted) to the total integrated overtone emission, and in the multilayer case these states were observed directly by the collection of dispersed fluorescence.…”