Rotational Excitation and Vibrational Relaxation ofH2(υ=1,J

Abstract

We have observed efficient translational to rotational energy transfer for H 2 ͑y 1, J 2 √ 0͒ scattered from Cu(111), the cross section increasing rapidly from near threshold to reach a maximum at a translational energy of 140 meV. Above this energy a change in the behavior of the H 2 ͑y 1, J 2 √ 0͒ cross section was seen, coinciding with the abrupt onset of H 2 ͑y 1, J͒ removal by coupling of translational and vibrational coordinates. Vibrational relaxation competes with rotational excitation, both processes occurring preferentially at the same impact sites and geometry. [S0031-9007 (97)02334-X] PACS numbers: 79.20.Rf, 68.35.JaThe potential energy surfaces (PES) and dynamics of activated dissociative chemisorption have become the focus of much attention recently as experiments become available which provide dynamical insight in a relatively direct way [1]. One source of information is energy transfer measurements which can be compared with scattering calculations to investigate the characteristics of the PES. Here we report state selected measurements of rotational energy transfer for H 2 at Cu(111) which is a model system of both experimental and theoretical interest [2]. Dissociation of H 2 at Cu(111) is highly activated and is enhanced by vibrational excitation of H 2 , the translational energy barrier to dissociation decreasing with increasing vibrational state [1,3,4]. This state dependence arises from the location of the barrier to dissociation at extended H-H separations in the exit channel leading to separated atoms. Calculations of the dissociation dynamics on low dimensional model potential energy surfaces [2] show sticking probabilities which depend on vibrational level, the sticking probability following an "S" shaped curve with a rapid increase in sticking as some threshold translational energy is reached. This picture is supported both by deconvolution of the sticking probabilities of individual vibrational levels from seeded beam measurements [4] and by application of microscopic reversibility to recombinative desorption data [1]. Strong coupling between the vibrational and translational coordinates is also evident for H 2 (D 2 ) scattering from Cu(111) [5,6]. For D 2 vibrational relaxation of ͑y 1, J͒ occurs at 0.35 eV, while at translational energies above 0.65 eV efficient vibrational excitation of ground state molecules to the excited vibrational level occurs. For D 2 it was shown that the vibrational excitation process scaled with normal energy [5], while the gap between the translational energy thresholds corresponds to the energy separation D 2 ͑y 1 √ y 0͒. Applying microscopic reversibility implies that the lower threshold is indeed due to T √ V transfer and cannot be attributed simply to dissociation. For H 2 ͑y 1͒ Rettner, Auerbach, and Michelsen observed a loss of flux near 0.3 eV, and vibrational excitation ͑y 1 √ y 0͒ was again seen at energies above 0.6 eV, just above the threshold for dissociative chemisorption of the ground state [6]. Scattering calculations on model two dimensiona...

Diffractive and reactive scattering of (v=0, j=0) HD from Pt(111): Six-dimensional quantum dynamics compared with experiment

“…III A, rotational excitation of HD is expected to be more efficient than that of H 2 because HD is asymmetric and has more closely spaced energy levels. An additional explanation for these large rotational excitation probabilities at low collision energies has been suggested in previous theoretical 17,33,51 and experimental 7,17 studies. Because the threshold energy to reaction is equally low, molecules are able to approach the barrier closely at these low energies, where they experience a high anisotropy of the potential energy, allowing efficient rotational excitation.…”

“…Since the early 1980s, the reactive, the rotationally elastic and inelastic, and the diffractive scattering of molecular hydrogen from metal surfaces have been studied extensively, both experimentally [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] and theoretically. [17][18][19][20][21][22][23][24][25][26][27][28] Much attention has been devoted to HD scattering from Pt͑111͒ because of, inter alia, the high probabilities for rotational excitation to only a limited number of accessible rotational states.…”

Reactive and diffractive scattering of H2 from Pt(111) studied using a six-dimensional wave packet method

“…Previous theoretical 57,58 and experimental 58,59 results show that large rotational excitation probabilities occur for collision energies E Z close to the threshold energy to reaction, because the molecule is able to come close to the barrier where the potential contains a large amount of anisotropy. This explains why rotational excitation is found already for low E Z ; the lowest barrier in the PES to reaction is only 0.06 eV.…”

Vibrational de-excitation of v=1 H2 during collisions with a Cu(100) surface

“…30 Perhaps, the experimental configuration used in the associative desorption experiments can be modified to include an H 2 molecular beam, with the use of stimulated Raman pumping to overpopulate (vϭ1, jϭ0) in the incident molecular beam. [17][18][19] The stimulated Raman pumping technique has already been used in state-to-state molecular beam experiments measuring rovibrationally inelastic scattering from (vϭ1, jϭ1) to (vЈϭ0, jЈ) states of H 2 on Pd͑111͒ ͑Ref. 19͒ and on Cu͑100͒ ͑Ref.…”

“…The rotational-state distribution and orientational an-isotropy of scattered molecules are explained in terms of features of the potential energy surface. Recent experimental advances, including the ability to overpopulate specific rovibrational states in molecular beams and to perform final-state population analysis, [17][18][19] offer the possibility for detailed state-to-state comparisons between the presented theoretical predictions and experimental observations yet to be performed. Some of the results we will present for the scattering of ͑vϭ1, jϭ1) H 2 can be compared to existing experimental results.…”