Internal State Distribution Research Articles

Probing the dynamics of weakly bound complexes using high-resolution laser spectroscopy

A combination of high-resolution infrared spectroscopy and photofragment detection methods has been used to study the vibrational predissociation of a number of weakly bound complexes at the state-to-state level. The goal of this research is to provide detailed information on the dissociation of these highly non-statistical systems that can be used to make comparisons with the results of emerging theoretical methods and to gain new insights into the nature of bond rupture. In this paper we discuss results for several systems which illustrate some of the new methods we have developed. In the case of the HF dimer, the photofragment angular distributions provide sufficient information to assign the internal state distributions of the fragments. The experimental results are compared with recently reported theoretical calculations. Preliminary experimental results are also reported for the HF–DF and DF–HF complexes which, when combined with previously reported theoretical calculations and the HF dimer results, reveal deficiencies in the existing potential-energy surfaces that are not evident from the available spectroscopy.For systems where the fragments have smaller rotational constants or accessible excited vibrational states, the angular distributions no longer provide a unique assignment of the internal state quantum numbers. Without such, the detail with which the photofragmentation process can be studied is greatly reduced. For this reason we have developed two new methods that greatly aid in determining such information. The first involves the use of a second infrared laser to probe the fragments spectroscopically, while the second method involves orienting the parent molecular complex using a large dc electric field prior to dissociating it. In this way the two fragments can be detected separately, reducing the congestion in the angular distribution. These methods will be illustrated with several examples.

Internal state populations and the time-of-flight of ground-state species ejected after the 193 nm excimer laser ablation of CuO, BaO2, and Y2O3

Laser-induced fluorescence is used to measure time-of-flight (TOF) and internal state distributions of atoms, ions, and diatomics ejected after the 193 nm ablation of Y2O3, BaO2, and CuO. These measurements indicate that the bulk of material is ejected with speeds in the range of 104–105 cm/s while particle speeds in the emitting component of the plume exceed 106 cm/s. The TOF profiles of all the species were non-Maxwellian, containing extended low-velocity ‘‘tails.’’ It is postulated that these tails arise from the evaporation and/or sputtering of target material that occurs after the laser ablation pulse. This particle emission is caused either by residual energy deposited in the target after the explosive vaporization/ablation pulse or from radiation exchange and/or ion bombardment from the expanding plasma plume. The extent of these ‘‘tails’’ increases with increasing fluence, generating oscillations in the TOF distributions of Cu atoms from CuO at high fluence. Rotational and vibrational temperatures were estimated for YO and BaO molecules in the plume, and no CuO was detected.

The search for direct vibrational excitation in gas–surface collisions of CO with Au(111)

I have examined the dynamics of energy transfer in the CO/Au(111) system to determine the probability of direct vibrational excitation. In contrast to earlier studies of NO/Ag(111), NH3/Au(111), and H2/Cu(111), no direct vibrational excitation is observed. Measurements have been made using molecular beam techniques to control the collision energy Ei and angle θi and using laser ionization detection to probe the internal state distributions of the scattered molecules. The probability of direct vibrational excitation is found to remain below 10−3 for Ei up to 1.4 eV at surface temperatures Ts of 300 and 800 K and θi=10°. I have also determined the probability of deexcitation for CO(v=1) colliding with Au(111). Within the uncertainties (∼20%), no deexcitation is apparent for Ei in the range 0.2–1.1 eV with Ts=300 K. With Ts=800 K and Ei=1.1 eV, the survival probability was also indistinguishable from unity. As part of this study, I have determined the variation of the trapping probability of CO on Au(111) as a function of kinetic energy for different surface temperatures, and I report rotational distributions for scattered CO for Ei of 0.5, 0.9, and 1.4 eV with θi=10°. In contrast to the weak coupling to molecular vibration, a high degree of rotational excitation is observed, yielding pronounced rotational rainbows. Using beams with ∼1% v=1 populations, I find that the rotational distributions of scattered v=1 molecules are the same as those for scattered v=0 within the uncertainties of the measurements.

Nascent rotational and vibrational distributions in both products of the reaction Zn(4 1P1)+H2O→ZnH(X 2Σ+)+OH(X 2Π)

The reaction Zn(4 1P1)+H2O→ZnH(X 2Σ+)+OH(X 2Π) was studied under thermal equilibrium conditions at 700 K. The nascent internal state distributions of both products ZnH and OH were determined by using a pump-and-probe technique. The rotational distributions of ZnH and OH were both Boltzmann-like for their v″=0 vibrational levels. However, the rotational temperatures were significantly different—12 000 K for ZnH and 900 K for OH. ZnH was also vibrationally excited. The nascent vibrational distribution of ZnH was determined to be 10(v″=0):13(v″=1):7(v″=2):2(v″=3). In contrast, no excitation in the OH vibration was observed. Such a nonstatistical energy partitioning is explained by considering a short-lived Zn–H–OH intermediate in a nonlinear geometry.

Quantum dynamics studies of adsorption and desorption of hydrogen at a Cu(111) surface

The quantum dynamics of dissociative adsorption and associative desorption of hydrogen on Cu(111) surface over an atop site has been studied in detail using the S-matrix Kohn variation method for reactive scattering. We employed an empirical London–Erying–Polanyi–Sato (LEPS) type potential energy surface (PES) with parameters fitted to the available experimental adsorption data and to theoretical cluster calculations. The dissociation probability of hydrogen, as a function of normal kinetic energy, is calculated for individual rovibrational states with the v=1 translational energy threshold being lower than that of v=0 by about 0.317 eV. Our calculation shows that dissociative adsorption of H2 on Cu(111) at relatively low kinetic energies (<0.4 eV) is dominated by the component of vibrationally excited H2(v=1), whereas ground H2(v=0) plays the dominate role at higher kinetic energies. In addition to vibrational enhancement of hydrogen dissociation, the role of hydrogen rotation in dissociative adsorption has also been examined. In particular, in-plane rotation of H2(m=j) is found to be more favorable for dissociation than out-of-plane rotation (m=0), similar to the finding from a previous study on H2/Ni(111) system. The present study also examined internal state distributions of H2 desorbed from Cu(111). The vibrational population ratio Pv=1/Pv=0 in desorption is much larger than the thermal distribution at surface temperatures. The relation between the vibrational population ratio in desorption and the vibrational enhancement in adsorption is discussed and analyzed. Our theoretical results are compared to the recent experimental results for both adsorption and desorption of H2 on Cu.

Effect of rotation on the translational and vibrational energy dependence of the dissociative adsorption of D2 on Cu(111)

We have investigated the dependence on the rotational and vibrational states of the translational energy of D2(v,J) formed in recombinative desorption from Cu(111). These results provide information about the effect of rotational energy relative to that of vibrational and translational energy on the dissociative chemisorption of D2 on Cu(111). The range of rovibrational states measured includes rotational states J=0–14 for vibrational state v=0, J=0–12 for v=1, and J=0–8 for v=2. D2 molecules were detected in a quantum-state-specific manner using three-photon resonance-enhanced multiphoton ionization (2+1 REMPI). Kinetic energies of desorbed molecules were obtained by measuring the flight time of D2+ ions in a field-free region. The mean kinetic energies determined from these measurements depend strongly on the rotational and vibrational states. Analyzing these results using the principle of detailed balance confirms previous observations that vibrational energy is effective, though not as effective as translational energy, in promoting adsorption. Rotational motion is found to hinder adsorption for low rotational states (J≤5) and enhance adsorption for high rotational states (J≥5). Even for high J states, however, rotational energy is less effective than either vibrational energy, which is 30%–70% more effective than rotational energy, or translational energy, which is 2.5–3 times more effective than rotational energy in promoting adsorption. The measured internal state distributions for the rovibrational states listed above are consistent with the observed dependence of the kinetic energy of the de- sorbed molecules with the rotational state. In addition, the analysis performed yields the dependence of the adsorption probability on kinetic energy separately for each rovibrational state. These functions have very similar sigmoidal shapes for all states examined. Changing the quantum state is primarily associated with a shift in the position, or threshold energy, for the curves. The level at which these functions saturate or level off at high energy is independent of rotational state but varies nonmonotonically with the vibrational state.

Integral rate constant measurements of the reaction H +D2O → HD(v′, j′)+OD

The reaction H+D2O was studied by intersecting a pulsed beam of HI with an effusive spray of D2O in a high vacuum chamber. Translationally hot H atoms were generated by UV photolysis of HI in the intersection volume, and the HD product of the reaction H+D2O was detected in a quantum-state-specific manner by (2+1) resonance-enhanced multiphoton ionization. Because the same UV laser beam was used to initiate the reaction and detect the product, the relative collision energy varied as a function of product state detected—∼2.8 eV for v′=0, ∼2.6 eV for v′=1, and ∼2.5 eV for v′=2. Under these conditions, approximately 35% of the available energy is partitioned into the internal modes of the HD product. For the products, the HD ‘‘new bond’’ receives 15 times more energy than the OD ‘‘old bond.’’ A significant amount of energy appears as HD vibration with v′=0 and 1 having comparable populations. The fraction of available energy partitioned into HD rotation, gR(v′), is found to be essentially independent of HD vibration. This invariance may be rationalized in terms of a counterbalancing of two mechanisms for rotational excitation of the HD product. We find qualitative agreement between recent quasiclassical trajectory calculations by Kudla and Schatz for the HD product internal-state distributions and the present experimental results.

Effect of reagent vibration on the hydrogen atom + water-d reaction: an example of bond-specific chemistry

OH and OD product branching ratios and internal state distributions have been measured in a state-to-state study of the reaction H+HOD(υ 1 ,υ 2 ,υ 3 )→OH(υ,N)(OD(υ,N))+HD(H 2 ). The HOD molecules are prepared in a specific vibrational level (one or two J states) by infrared excitation of thermal HOD using a tunable optical parametric oscillator. Fast («hot») H atoms are generated by laser photolysis of H1 at 266 nm, and the OH and OD reaction products are probed quantum-state-specifically by laser-induced fluorescence.

Interaction dynamics of hydrogen at a Cu(111) surface

We have used molecular beam and laser detection techniques to study the dynamics of adsorption and desorption of H 2 and D 2 at a Cu(111) surface. We have determined the adsorption probability for beams with kinetic energies up to 0.83 eV, vibrational temperatures up to 2300 K, and incidence angles from 0° to 60°. A detailed analysis of these data yields adsorption probability functions for molecules in individual vibrational states. We have also studied the internal state distribution of molecules scattered from the surface. These measurements provide state-resolved information on both adsorption and vibrational excitation. The adsorption and scattering studies have been complemented by studies of desorption dynamics, including preliminary measurements of the velocity distributions of molecules desorbed in specific quantum states. The desorption data are interpreted via the principle of detailed balance to give information on the temperature dependence of the adsorption probability functions. The vibrational dependence of the desorption velocity distributions reproduces trends predicted from the adsorption measurements.

Theoretical modeling of photodissociation dynamics of CH3I on LiF(001)

A new method is developed for the simulation of atoms and molecules interacting with ionic surfaces. This approach, based on a quasi-two-dimensional Ewald sum and a two-dimensional Fourier transformation, is capable of evaluating the long-range Coulomb interactions for a semi-infinite ionic solid. We have applied this method to investigate the photodissociation dynamics of CH3I on a LiF(001) surface. All the degrees of freedom of the adsorbed molecule are considered and the excited state dissociation potentials of CH3I are described by analytical functions derived from a recent ab initio calculation. The substrate (LiF) is represented by 6×6×3 movable atoms surrounded by static ions at their equilibrium positions. The adsorbate/substrate interaction is modeled as a sum of Coulomb and Lennard-Jones pairwise potentials. A phenomenological term is introduced to account for the adsorbate/adsorbate interaction. The equilibrium configurations of the system at a given temperature are obtained by a Monte Carlo method, which shows that there exist two stable configurations with the CH3I molecular axis perpendicular to the surface, either methyl up or down. The dissociation dynamics of the adsorbate is studied with a classical molecular dynamics method and the angular, kinetic energy, and rovibrational distributions of the fragments are calculated. When the molecule is adsorbed with the methyl up, the methyl fragment dissociates into the vacuum promptly with kinetic energy and internal state distributions similar to those in the gas phase. If the molecule is adsorbed with the methyl down, however, the methyl fragment could collide with iodine after rebounding from the surface, transferring a significant amount of kinetic energy to the iodine. A much broader and more energetic kinetic energy distribution of the iodine fragments is observed under such circumstances. The energy transfer is most effective when the parent molecule orients parallel to the surface normal and decreases as the angle deviates from this direction. We also observed a substantial increase in the rotational angular momentum of the methyl fragment and a cooler vibrational distribution for the umbrella mode as a result of the collision.

Scattering and dissociation of H2/D2 at Fe(110)

The dissociative chemisorption of H2/D2 at an Fe(110) surface has been studied as a function of the translational energy of the incident molecules. The sticking probability shows an activated behaviour, increasing with translational energy, similar to that previously reported1 but with a rather higher dissociation probability. The sticking coefficients for H2 and D2 are the same within experimental error. Seeding experiments at a constant translational energy indicate that adsorption on a clean surface is insensitive to the internal state distribution P(v, J) of the incident molecules, in contrast to measurements reported for the Fe(100) surface. The sticking probability does not depend on the surface temperature and deviates significantly from a normal energy scaling, momentum parallel to the surface inhibiting dissociation. Sticking is dramatically inhibited by low concentrations of adsorbed O and dissociation becomes sensitive to the internal energy of the incident molecules. Under these conditions the scattered flux of H2(v, J) shows a steady change in reflectivity with translational energy, similar to the steady variation in the overall sticking coefficient. There is no sign of any abrupt translational energy threshold for removal of this vibrational level. The sticking on the clean and O precovered surfaces is discussed in the light of the recent results showing the influence of internal energy on dissociation.

Photodissociation of nitrogen dioxide adsorbed on lithium fluoride (001)

The photochemistry of NO2 physisorbed on single-crystal LiF(001) at 100 K has been studied at lambda1 = 248 nm. The adsorbate was examined by polarized FTIR in both the presence and absence of lambda1 radiation. In the absence of UV irradiation the adlayer is composed of dimeric (NO2)2. In the presence of UV, FTIR shows that some N2O3 is formed. Photodissociations (PDIS) giving both NO(g) and molecular NO2(g) were the predominant mechanisms as determined by time-of-flight mass spectrometry (TOF-MS) and resonantly enhanced multiphoton ionization (REMPI). The main objective of this work was the characterization of the photoproduct, NO, internal state distribution by 1 + 1 REMPI. Vibrational levels from v'' = 0 to 9 were probed with rotational resolution using a tunable laser, lambda2. The rotational distributions for each vibrational level could be described by one Boltzmann temperature. The spin-orbit states of NO(g) were equally populated in all vibrational levels. The lambda doublet states, PI(A') and PI(A''), were equally populated. The principal observation was that the vibrational distribution in NO(g) was inverted and bimodal with a peak in v'' = 0 and a second substantial peak in v'' = 3-4, qualitatively resembling but quantitatively different from that for photolysis of NO2(g). Time delays between the two lasers were used to probe the translational energy of the NO(g) photofragment in specified states of internal excitation. The translational energy distributions were invariant over all vibrational levels, except v'' = 0 for which much slower fragments were observed. This complete determination of the energy distribution in the degrees of freedom of the NO(g) from photodissociation of adsorbate has implications for the identity of the photolyzing species and the dynamics of photodissociation. Two mechanisms for photoformation of NO2(g) were found: one at low coverages and one at higher coverages, the former giving peak translational energies approximately 1.2 kcal/mol and the latter 0.4 kcal/mol.

Chemistry in strong laser fields: An example from methyl iodide photodissociation

Time-dependent quantum-mechanical theories and simulations provide a clear and intuitive description of molecular processes. Due to ensuing simplification of the theory and the generally employed numerical algorithms, the vast majority of these treatments are based upon perturbation theory. Especially in light of the current level of experimental sophistication, with experiments being realized which are influenced by the spectral, temporal, and spatial shape of the laser pulse, it is important to move beyond treatments limited to weak fields or idealized δ-function wave forms. Various methods to examine the results of high-field simulations are presented. All of the techniques are shown to have the familiar linear response form in the weak-field limit. In a time-dependent framework the difference between the linear and nonlinear response expressions can be seen from expectation values over stationary versus nonstationary states. The high-field photodissociation of methyl iodide illustrates this approach. Methyl iodide represents a physical system well suited for examining the effects of such exciting laser-field characteristics as strength, linewidth, and frequency upon the photodissociation dynamics. Its dissociation occurs upon coupled repulsive excited electronic potential-energy surfaces which have recently been revised to fit the most current experimental data. The effect of the surface intersection has previously been typically studied by examining the branching and the internal state distributions of the products in the two channels as a function of excitation frequency only. The collinear photodissociation dynamics is examined using a numerically exact time-dependent quantum-mechanical method. The equations of motion for the amplitudes upon the ground and two coupled excited electronic surfaces, explicitly incorporating the laser field, are integrated by a scheme which employs a low-order polynomial approximation to the evolution operator. The effects of the three field characteristics upon the branching ratio and internal state distributions of the products and the spectroscopy of the process are delineated. The course of the photodissociation dynamics is shown to be affected by these characteristics. The results demonstrate the causal connections between the pulse shape and the resulting photoprocesses. Practical manifestations of strong fields (power broadening, sub-threshold absorption, higher harmonic generation, emission shaping of the ground state, temporal development) are stressed.

Quasiclassical trajectory simulation of the kinematically constrained reaction Ba+HI→BaI+H

We report a quasiclassical study of the kinematically constrained reaction Ba+HI→BaI+H. A London–Eyring–Polanyi–Sato (LEPS) potential is constructed based on spectroscopic data for BaI, BaH, and HI with its Sato parameters adjusted to reproduce the experimentally determined value of the maximum impact parameter for BaI(v=0). The main purpose of the study is to guide future experiments by testing how sensitive different features are to this mass combination. Under the conditions used in the previous Ba+HI crossed-beam experiment, we find that (1) the specific opacity function Pv(b) for a vibrational level v is narrow and overlaps those from neighboring v’s; (2) the most probable impact parameter for reaction producing a given vibrational level decreases as v increases (v≤12); (3) the initial orbital angular momentum is mapped onto BaI rotational angular momentum with a spread of ±19ℏ; (4) the recoil energy has a broad, featureless distribution that is nearly the same for different v levels, and shows no strong correlation with the initial relative velocity, vrel. Product internal-state distributions, recoil angular distributions, and isotope effects are also presented.

Photodissociation dynamics of CH3I adsorbed on MgO(100): Theory and experiment

Theoretical and experimental results are compared for the 257 nm photolysis of methyl iodide adsorbed on an MgO(100) crystal. Molecular-dynamics calculations treat CH3I as a pseudodiatomic molecule and describe the geometry and the vibrational and librational frequencies of ground state CH3I on the surface of a solid at 125 K. The simulations modeled the photodissociation dynamics of the adsorbed species. The photoexcitation of CH3I at 257 nm is to the 3Q0 state which is, in turn, coupled to the 1Q1 state. The electronic surface coupling allows for two dissociation pathways, producing either ground- or excited-state iodine atoms in concert with ground-state methyl radicals. The I*/I branching ratio and the velocity and angular distributions of both photofragments are predicted by the theory. A comparison is made between these predictions and experimental observation of the I*/I branching ratio, the velocity distribution of the methyl fragment, and the internal state distribution of the methyl. A substantial lowering of the I*/I ratio as compared to data from the gas-phase photodissociation studies is both predicted by theory and seen experimentally. Theoretical simulations attribute this change to efficient trapping of the I* photofragments by the surface. Further comparisons between the theoretical predictions and the experimental data support a model where the molecule is aligned perpendicular to the surface and the escape of iodine atoms from the surface following the photodissociation of adsorbed methyl iodide involves collisions with the surface.

Photodissociation of BrCH2CH2OH and ICH2CH2OH: Formation and characterization of OH(X 2Π)

This work studies the dissociation of BrCH2CH2OH and ICH2CH2OH following laser excitation to the electronic à state. The direct subpicosecond dissociation cleaves the C–X bond to produce CH2CH2OH and the halogen atom in either the ground (2P3/2) or excited (2P1/2) state. The initial dissociation produces excited CH2CH2OH in a distribution of internal states, some of which have sufficient energy to undergo secondary, nonphotoinduced dissociation to form OH and C2H4. We probe the OH translational and rotational product states following this secondary dissociation using laser-induced fluorescence in both flow cell and pulsed jet apparatus. Excitation of BrCH2CH2OH at 202 nm produces OH with no vibrational excitation and a non-Boltzmann rotational distribution with an average rotational energy of 8 kJ/mol. Doppler analysis of the rotational line shape shows that a majority of the energy available for the secondary dissociation is partitioned into relative translation of OH and C2H4. Excitation of ICH2CH2OH at 266 nm also produces OH, but at much lower quantum yields due to energetic constraints. We find that the CH2CH2OH radical also absorbs at 266 nm to produce OH. The rotational and translational energy distribution of OH from secondary dissociation of the CH2CH2OH radical is described qualitatively by a simple rotational model.

Internal-state distributions of H2 desorbed from mono- and dihydride species on Si(100)

Following adsorption of atomic hydrogen on Si(100)–(2×1), the surface is heated and the desorbed H2 is detected via (2+1) resonance-enhanced multiphoton ionization (REMPI). H2 desorption correlated with the decomposition of dihydride groups on the surface (SiH2) is detected at a surface temperature Ts near 660 K, and with the monohydride species (SiH) near Ts=780 K. Although the H2 rotational distributions are nearly identical for the mono- and dihydride species, the vibrational distributions differ with roughly 0.2% and 1% of the population in H2(v=1) for the monohydride and dihydride, respectively. The enhancement in the [H2(v=1)]/[H2(v=0)] population ratio over that of a thermal distribution at Ts is, however, roughly 20 times for both mono- and dihydride species. The results are interpreted within a model that assumes desorption proceeds through a common intermediate, which is identified as the dihydride.

Recombinative desorption of H2 on Si(100)-(2×1) and Si(111)-(7×7): Comparison of internal state distributions

The dynamics of recombinative hydrogen desorption from the Si(100)-(2×1) and Si(111)-(7×7) surfaces have been compared using (2+1) resonance-enhanced multiphoton ionization to probe the desorbed H2. After dosing the surface with disilane (Si2H6), we performed temperature programmed desorption in a quantum-state-specific manner. The rovibrational-state distributions of H2 desorbed from both Si(100)-(2×1) and Si(111)-(7×7) are found to be the same within experimental accuracy. The rotational distribution is non-Boltzmann and has an average energy significantly lower than kTs, where Ts is the surface temperature. In contrast, superthermal energy is observed in the vibrational degree of freedom, and the v=1 to v=0 population ratio is approximately 20 times higher than that predicted by Boltzmann statistics. Our results imply that the details of the recombinative desorption process that affect the product state distribution are remarkably insensitive to the structural differences between the surfaces. We suggest that the transition-state geometry is similar on both surfaces and propose a model for hydrogen recombinative desorption localized at a single silicon atom.

Determination of the internal state distribution of NO(X 2Π) produced in the O(3P)+NH(X 3Σ−) reaction

The internal state distribution of the NO product from the O(3P)+NH(X 3Σ−) reaction has been determined from a laser fluorescence experiment in a cell at a total pressure of 60 mTorr. The O atom and the NH reagents were prepared in a microwave discharge in oxygen and by the two-photon 193 nm photolysis of ammonia, respectively. The NO product was observed in the vibrational levels v=1–8 by laser fluorescence excitation in A 2Σ+–X 2Π bands. The nascent vibrational state distribution was found to be monotonically decreasing vs increasing v. The v=1 rotational state distribution, extrapolated back to zero photolysis-probe delay, could be parametrized as a 1130±50 K Boltzmann distribution. Very little of the available energy is found as internal excitation of the NO product. The O+NH→H+NO reaction is expected to proceed by the formation and decay of a short-lived HNO complex. The observed NO vibrational state distribution is interpreted in terms of a Franck–Condon model involving the overlap of vibrational wave functions for the NO stretch coordinate in the HNO complex with those for vibration in the free NO product. The NO rotational state distribution is governed largely by kinematic constraints in this H+HL→HH+L reaction, where H and L are heavy and light atoms, respectively.

Theoretical model for the dynamics of hydrogen recombination on the Si(100)-(2×1) surface

We propose in this paper a quantitative theoretical model to describe the recombination dynamics of hydrogen on Si(100)-(2×1) surface. The desorption kinetics of hydrogen on Si(100) has been experimentally determined to obey a first-order rate law and the internal state distributions of desorbed hydrogen has recently been determined experimentally using the resonantly enhanced multiphoton ionization technique. In this theoretical model, which has the characteristic of preassociative desorption, the rate of desorption and the internal state distribution of H2 is given by a thermally averaged golden-rule expression. In particular, the desorption of H2 is supposed to result from a bound-free transition between an initially bound state composed of two H–Si dangling bonds on the same silicon dimer and a final continuum state consisting of H2 plus Si surface. In addition to explaining the first-order desorption kinetics, our model dictates that H2 will be vibrationally hot upon desorption, whereas rotation of H2 will generally be expected to be cold because of symmetry constraints. These conclusions about the dynamics are consistent with recent experiments [K. W. Kolasinski, S. F. Shane, R. N. Zare, J. Chem. Phys. 95, 5482 (1991); 96, 3995 (1992)] in which hydrogen is found to be vibrationally excited but rotationally cold. We show, in this paper, that essentially all recent experimental results on hydrogen desorption on Si(100) can be qualitatively explained based on the Franck–Condon factors in our model. A co-planar model calculation is carried out using our method, and the calculated rovibrational distribution of H2 is compared with the aforementioned experimental results of Kolasinski, Shane, and Zare.