Primary photodissociation mechanisms of pyruvic acid on S1: observation of methylhydroxycarbene and its chemical reaction in the gas phase

Following S₁ ← S₀ excitation at 351 nm, pyruvic acid dissociates mainly into methylhydroxycarbene (MHC) and CO₂. Some MHC molecules isomerize to more stable acetaldehyde and vinyl alcohol; the remaining MHC is stabilized and reacts bimolecularly.

Temperature dependence of the photodissociation of CO2 from high vibrational levels: 205-230 nm imaging studies of CO(X1Σ+) and O(3P, 1D) products

The 205-230 nm photodissociation of vibrationally excited CO₂ at temperatures up to 1800 K was studied using Resonance Enhanced Multiphoton Ionization (REMPI) and time-sliced Velocity Map Imaging (VMI). CO₂ molecules seeded in He were heated in an SiC tube attached to a pulsed valve and supersonically expanded to create a molecular beam of rotationally cooled but vibrationally hot CO₂. Photodissociation was observed from vibrationally excited CO₂ with internal energies up to about 20 000 cm^-1, and CO(X¹Σ⁺), O(³P), and O(¹D) products were detected by REMPI. The large enhancement in the absorption cross section with increasing CO₂ vibrational excitation made this investigation feasible. The internal energies of heated CO₂ molecules that absorbed 230 nm radiation were estimated from the kinetic energy release (KER) distributions of CO(X¹Σ⁺) products in v″ = 0. At 230 nm, CO₂ needs to have at least 4000 cm^-1 of rovibrational energy to absorb the UV radiation and produce CO(X¹Σ⁺) + O(³P). CO₂ internal energies in excess of 16 000 cm^-1 were confirmed by observing O(¹D) products. It is likely that initial absorption from levels with high bending excitation accesses both the A¹B₂ and B¹A₂ states, explaining the nearly isotropic angular distributions of the products. CO(X¹Σ⁺) product internal energies were estimated from REMPI spectroscopy, and the KER distributions of the CO(X¹Σ⁺), O(³P), and O(¹D) products were obtained by VMI. The CO product internal energy distributions change with increasing CO₂ temperature, suggesting that more than one dynamical pathway is involved when the internal energy of CO₂ (and the corresponding available energy) increases. The KER distributions of O(¹D) and O(³P) show broad internal energy distributions in the CO(X¹Σ⁺) cofragment, extending up to the maximum allowed by energy but peaking at low KER values. Although not all the observations can be explained at this time, with the aid of available theoretical studies of CO₂ VUV photodissociation and O + CO recombination, it is proposed that following UV absorption, the two lowest lying triplet states, a³B₂ and b³A₂, and the ground electronic state are involved in the dynamical pathways that lead to product formation.

Amorphous Solid Water Films: Transport and Guest−Host Interactions with CO2 and N2O Dopants

Guest-host interactions have been examined experimentally for amorphous solid water (ASW) films doped with CO2 or N2O. The main diagnostics are Fourier transform infrared (FTIR) spectroscopy and temperature programmed desorption (TPD). ASW films deposited at 90 K are exposed to a dopant, and the first molecules that attach to a film enter its bulk until it is saturated with them. Subsequent dopant adsorption results in crystal growth atop the ASW film. There are distinct spectral signatures for these two cases: LO and TO vibrational modes for the crystal overlayer, and an easily distinguished peak for dopant molecules that reside within the ASW film. Above 105 K, the dopant surface layer desorbs fully. Some dopants residing within the ASW film remain until 155 K, at which point the ASW-to-crystalline-ice transition occurs, expelling essentially all of the dopant. No substantial differences are observed for CO2 versus N2O. It is shown that annealing an ASW film to 130 K lowers the film's capacity to include dopants by a factor of approximately 3, despite the fact that the ASW spectral feature centered at approximately 3250 cm(-1) shows no discernible change. Sandwiches were prepared: ASW-dopant-ASW etc., with the dopant layer displaying crystallinity. Raising these samples past 105 K resulted in the expulsion of essentially all of the crystalline dopant. What remained displayed the same spectral signature as the molecules that entered the bulk following adsorption at the surface. It is concluded that the adsorption sites, though prepared differently, have a lot in common. Dangling OH bonds were observed. When they interacted with a dopant, they underwent a red shift of approximately 50 cm(-1). This is in qualitative agreement with studies that have been carried out with weakly bound binary complexes. As a result of this study, a fairly complete, albeit qualitative, picture is in place for the adsorption, binding, and transport of CO2 and N2O in ASW films.

Femtosecond Multidimensional Imaging of a Molecular Dissociation

The coupled electronic and vibrational motions governing chemical processes are best viewed from the molecule's point of view-the molecular frame. Measurements made in the laboratory frame often conceal information because of the random orientations the molecule can take. We used a combination of time-resolved photoelectron spectroscopy, multidimensional coincidence imaging spectroscopy, and ab initio computation to trace a complete reactant-to-product pathway-the photodissociation of the nitric oxide dimer-from the molecule's point of view, on the femtosecond time scale. This method revealed an elusive photochemical process involving intermediate electronic configurations.

Photoinitiated Predissociation of the NO Dimer in the Region of the Second and Third NO Stretch Overtones

Photofragment yield spectra and NO(X(2)Pi(1/2,3/2); v = 1, 2, 3) product vibrational, rotational, and spin-orbit state distributions were measured following NO dimer excitation in the 4000-7400 cm(-1) region in a molecular beam. Photofragment yield spectra were obtained by monitoring NO(X(2)Pi; v = 1, 2, 3) dissociation products via resonance-enhanced multiphoton ionization. New bands that include the symmetric nu(1) and asymmetric nu(5) NO stretch modes were observed and assigned as 3nu(5), 2nu(1) + nu(5), nu(1) + 3nu(5), and 3nu(1) + nu(5). Dissociation occurs primarily via Deltav = -1 processes with vibrational energy confined preferentially to one of the two NO fragments. The vibrationally excited fragments are born with less rotational energy than predicted statistically, and fragments formed via Deltav = -2 processes have a higher rotational temperature than those produced via Deltav = -1 processes. The rotational excitation likely derives from the transformation of low-lying bending and torsional vibrational levels in the dimer into product rotational states. The NO spin-orbit state distribution reveals a slight preference for the ground (2)Pi(1/2) state, and in analogy with previous results, it is suggested that the predominant channel is X(2)Pi(1/2) + X(2)Pi(3/2). It is suggested that the long-range potential in the N-N coordinate is the locus of nonadiabatic transitions to electronic states correlating with excited product spin-orbit states. No evidence of direct excitation to electronic states whose vertical energies lie in the investigated energy region is obtained.

Two-photon dissociation of the NO dimer in the region 7.1–8.2 eV: Excited states and photodissociation pathways

A study of excited states of the NO dimer is carried out at 7.1-8.2 eV excitation energies. Photoexcitation is achieved by two-photon absorption at 300-345 nm followed by (NO)(2) dissociation and detection of electronically excited products, mostly in n=3 Rydberg states of NO. Photoelectron imaging is used as a tool to identify product electronic states by using non-state-selective ionization. Photofragment ion imaging is used to characterize product translational energy and angular distributions. Evidence for production of NO(A (2)Sigma(+)), NO(C (2)Pi), and NO(D (2)Sigma(+)) Rydberg states of NO, as well as the valence NO(B (2)Pi) state, is obtained. On the basis of product translational energy and angular distributions, it is possible to characterize the excited state(s) accessed in this region, which must possess a significant Rydberg character.

Competitive pathways via nonadiabatic transitions in photodissociation

Photodissociation processes of molecules and radicals involving multiple pathways and nonadiabatic crossings are studied using the photofragment imaging technique and the core-sampling version of time-of-flight spectroscopy. Capabilities and challenges are illustrated by two systems. The isocyanic acid system demonstrates how interactions among potential energy surfaces can change during dissociation. The hydroxymethyl photodecomposition system highlights Rydberg-valence interactions common in free radicals. The cross-fertilization between theory and experiment is emphasized.

Photolysis of ClNO adsorbed on MgO(100)

The 365 nm pulsed laser photolysis of nitrosyl chloride adsorbed on a rough MgO(100) surface at 90 K has been studied. Mass spectrometric detection was used to record time-of-flight (TOF) spectra by monitoring Cl+ and NO+. These ions can derive from parent ClNO, which fragments completely in the mass spectrometer, as well as from Cl and NO photofragments. The TOF distributions are considerably slower than for the corresponding gas phase photodissociation process. NO was also detected state selectively by using resonance enhanced multiphoton ionization (REMPI), and a channel corresponding to direct adsorbate photolysis was identified. The state selective detection of NO molecules that emerge from the surface following photolysis shows unambiguously that their rotational degrees of freedom reflect the surface temperature (Trot = 100−140 K), even at low coverages. At similar photolysis wavelengths, gas phase ClNO photodissociation is known to produce highly rotationally excited NO with a distinctive non-statistical distribution peaked at J″ = 46.5. Our studies suggest that, contrary to the gas-phase photolysis results, Cl and NO are not ejected rapidly following photolysis of surface-bound species on a repulsive potential energy surface. We postulate that ClNO grows in islands, with MgO defect sites serving as nucleation centers. Photofragment rotational and translational excitations are quenched efficiently due to strong attractive interactions that equilibrate NO to the surface temperature. Desorption of intact ClNO may also take place, but following (i.e., not during) the photolysis pulse. Such desorbed species can contribute to the TOF spectra, but not the REMPI spectra.