Indirect Photodissociation Research Articles

Unveiling Ultrafast-Weak-Field Coherent Control of Indirect Dissociation Reactions.

Coherent control of molecular photodissociation through one-photon transitions has become a topic of interest in physical chemistry. Previous studies have shown that modulating the spectral phase of a single ultrafast laser pulse while keeping its spectral amplitude constant does not affect the dissociation yield of reactions originating from a pure eigenstate of the ground electronic state. Here, we explore the indirect photodissociation reaction of NaI molecules using theoretical and numerical methods. Our findings show that, in contrast to the outcomes achieved with negatively chirped pulses, time-dependent population of the eigenstates of the excited adiabatic potential induced by positively chirped laser pulses, acting as intermediates in the reaction, cannot be periodically restored to that caused by the unchirped pulse. This gives rise to an intriguing phenomenon: the sign of the pulse's chirp rate influences the distribution of dissociation fragments in coordinate and momentum space over extended periods. This work highlights the potential of using spectral-phase modulated pulses to manipulate indirect photodissociation reactions, offering a way to modify the transient photofragment distributions by controlling reaction intermediates.

Intersecting conical intersection seams in tetra-atomic molecules: the S1–S0 internal conversion in HNCO

Symmetry-allowed intersections of two states of different symmetry, different-symmetry intersections, are the class of conical intersection most commonly studied in small molecules. A same-symmetry seam of conical interaction, originating from the same two states, may intersect this different-symmetry seam. These intersections of intersections are themselves continuously connected, forming a multidimensional subspace in the symmetry-allowed seam. A confluence of seams requires changes in the conical topography which can affect nuclear dynamics significantly. The indirect photodissociation of HNCO to H + NCO results from an internal conversion attributable to the portion of the S0-S1 seam with HNCO in a trans configuration. A previously introduced perturbative analysis is used to show that a manifold of confluences of conical intersections exists in this region. These confluences can influence the internal conversion dynamics and should be incorporated into a detailed description of this process. This represents the first time a dynamically relevant manifold of confluences has been demonstrated in a tetra-atomic molecule.

A test of the accuracy of the partially-separable time-dependent self-consistent-field approach

The accuracy of the time-dependent self-consistent-field (TDSCF) approach assuming partial factorization of the total wave packet is tested against an exact treatment, when applied to calculate asymptotic properties. The test is carried out in the framework of a three-dimensional simulation of the Ar–HCl UV photodissociation dynamics. All the partially-separable TDSCF ansatzs possible for this problem are investigated. The quality of the TDSCF results is found to be strongly dependent on the specific partially-separable ansatzs applied. In general, the TDSCF predictions are in very good (even quantitative) agreement with the exact ones for magnitudes associated with direct photodissociation dynamics, and are qualitative in the case of indirect photodissociation. The deviation of the TDSCF results from the exact dynamics is interpreted in terms of an error operator defined as the difference between the exact and the TDSCF Hamiltonians. The analysis of this operator also explains the different accuracy of the partially-separable ansatzs investigated. Based on this analysis, a simple procedure is suggested to estimate the relative average quality of the different TDSCF ansatzs.

Photodissociation of Ar–HCl: An energy-resolved study of the dynamics of total fragmentation into H+Ar+Cl

UV photolysis of Ar–HCl is simulated by means of an exact wave packet treatment in three dimensions. The focus of the work is on the mechanism of indirect dissociation of the hydrogen atom, which leads to total fragmentation of Ar–HCl into H, Ar, and Cl. The results predict for this photodissociation path a probability of about 13% of the photolysis process. The remaining probability would be associated with direct photodissociation of the H fragment. Kinetic-energy distributions of the hydrogen fragments produced by indirect photodissociation are calculated for different excitation energies of Ar–HCl. The distributions reflect a pronounced structure of peaks associated with broad and overlapping resonances of the system. The resonance structure is present in the whole energy range covered by the absorption spectrum. Hydrogen atoms initially populating the resonances can dissociate from the cluster extensively cooled down, after several collisions with Ar and Cl. A mechanism is suggested for the fragmentation process due to indirect photodissociation, which involves successive jumps of the hydrogen to lower-energy resonances, induced by the collisions. A classical collisional model is proposed to rationalize qualitatively the fragmentation dynamics.

Spectral Autocorrelation Function in Weakly Open Chaotic Systems: Indirect Photodissociation of Molecules

We derive the statistical limit of the spectral autocorrelation function and of the survival probability for the indirect photodissociation of molecules in the regime of non-overlapping resonances. The results are derived in the framework of random matrix theory, and hold more generally for any chaotic quantum system that is weakly coupled to the continuum. The "correlation hole" that characterizes the spectral autocorrelation in the bound molecule diminishes as the typical average total width of a resonance increases.

Three-dimensional infinite order sudden quantum theory for indirect photodissociation processes. Application to the photofragment yield spectrum of NOCl in the region of the T1(13A″) ←S(11A′) transition. Fragment rotational distributions and thermal averages

The analytical infinite order sudden (IOS) quantum theory of triatomic photodissociation, developed in paper I, is applied to study the indirect photodissociation of NOCl through a real or virtual intermediate state. The theory uses the IOS approximation for the dynamics in the final dissociative channels and an Airy function approximation for the continuum functions. The transition is taken as polarized in the plane of the molecule; symmetric top wave functions are used for both the initial and intermediate bound states; and simple semiempirical model potentials are employed for each state. The theory provides analytical expressions for the photofragment yield spectrum for producing particular final fragment ro-vibrational states as a function of the photon excitation energy. Computations are made of the photofragment excitation spectrum of NOCl in the region of the T1(13A″) ←S0(11A′) transition for producing the NO fragment in the vibrational states nNO=0, 1, and 2. The computed spectra for the unexcited nNO==0 and excited nNO=2 states are in reasonable agreement with experiment. However, some discrepancies are observed for the singly excited nNO=1 vibrational state, indicating deficiencies in the semiempirical potential energy surface. Computations for two different orientations of the in-plane transition dipole moment produce very similar excitation spectra. Calculations of fragment rotational distributions are performed for high values of the total angular momentum J, a feature that would be very difficult to perform with close-coupled methods. Computations are also made of the thermally averaged rotational energy distributions to simulate the conditions in actual supersonic jet experiments.

Three-dimensional analytical infinite order sudden quantum theory for triatomic indirect photodissociation processes

Our previously developed analytical infinite order sudden (IOS) quantum theory of triatomic photodissociation is extended to describe indirect photodissociation processes through a real or virtual intermediate state. The theory uses the IOS approximation for the dynamics in the final dissociative channels and an Airy function approximation for the continuum states. These approximations enable us to evaluate the multi-dimensional non-separable transition amplitudes analytically (as one-dimensional quadratures), despite the different natural coordinates for the initial bound, the intermediate resonant, and the final dissociative states. The fragment internal energy distributions are described as a function of the initial and final quantum states and the photon excitation energy. The theory readily permits the evaluation of rotational distributions for high values of the total angular momentum J in the initial bound molecular state, a feature that would be very difficult with close-coupled methods. In paper II we apply the theory to describe the photofragment yield spectrum of NOCl in the region of the T1(13A″)←S0(11A′) transition.

Photodissociation as a quantum transition: Photofragment vibrational distributions of C2N2(C̃ 1Πu) predissociation

Polyatomic indirect photodissociation is treated as a quantum transition between quasidiscrete and dissociative (photofragment) states. Our adiabatic method is followed to describe the nuclear dynamics of the dissociative state. Ab initio MCHF excited electronic potential energy surfaces are constructed and used to determine heavy-particle dynamics. The theory is applied to single-photon predissociation of C2N2(C̃ 1Πu) at 164, 158.7, and 153.6 nm to form CN(X 2Σ+)+CN(A 2Π). Theoretical predictions are found to be in good agreement with recent experimental product vibrational energy distributions.

Photodissociation processes in the CH molecule

The photodissociation processes in the CH molecule have been investigated using ab initio self-consistent-field with configuration-interaction (CI) methods. Potential energy curves of all states which dissociate into the lowest four atomic limits have been calculated, as well as those of several higher-lying states with Rydberg character. Transition dipole moments connecting the excited states with the lowest states have been obtained. Direct photodissociation of CH may occur by absorption into the continua of the B 2∑− and 2 2∑+ states, and cross sections for absorption from the X 2Π v″=0 state into these states are reported. Indirect photodissociation of CH may take place by absorption into bound states, which are subsequently dissociated through interactions with continuum states. The A 2Δ vibrational levels couple with the continuum of the X 2Π state through the nuclear rotational operator. The nuclear rotational coupling elements have been obtained from the CI wave functions, and predissociation rates are presented. The C 2∑+ v′=0 state is shown to be predissociated efficiently by spin-orbit coupling with the B 2∑− state. The 2, 3, and 4 2Π states of CH interact strongly through the radial nuclear kinetic energy operator. The nuclear radial coupling elements between these states have been computed by numerical differentiation of the CI wave functions. The coupled equations for the nuclear wave functions have been solved in a diabatic formulation. The calculated cross section for absorption into the coupled states is similar to that found recently for absorption into the 2 2Π and 3 2Π states of the OH molecule: it consists of a series of resonances with asymmetric Beutler–Fano profiles superimposed on a strong continuous background. The 3(F)2∑+ state, as well as the higher-lying (Rydberg) states, may interact with the 2Π states through rotational coupling or with the 2 4∑− state through spin-orbit coupling. Due to uncertainties in the coupling elements, the efficiencies with which these states are dissociated are not well determined, but they are probably small for most states. The photodissociation rate of CH by the unattenuated interstellar radiation field has been computed. The total rate is 9.5×10−10 s−1. The 2 2∑+ and coupled 2Π channels are most effective in the photodissociation of interstellar CH.

The photodissociation cross section of the A 1Π-X 1Σ + system of CH +

The A 1Π-X 1Σ + photodissociation cross section is calculated for temperatures of 0 and 300 K, including direct transitions to the continuum and indirect photodissociation via transitions to quasi-bound levels of the A 1Π state. The photodissociation rate at 0 K for the A-X system is also presented. The results are smaller than predicted previously.

The photodissociation of H 2S: A study of indirect photodissociation

We have performed calculations for the photodissociation of H 2S using surfaces constructed to test a model proposed by van Veen et al., in which the dissociation occurs via predissociated levels of the bound 1B 1 excited state. Total Franck-Condon factors for the photodissociation and partial Franck-Condon factors for the product vibrational distributions are presented.

Comment on the rotational state dependence of indirect photodissociation of a polyatomic molecule

The rotational state dependence of indirect photodissociation has been examined in the light of some recent developments in the theory of radiationless transitions in rotating molecules. A simple argument based on projection operators leads to the prediction of sequential kinetics from an initial singlet state through an intermediate triplet bound state and on to a final dissociative state. The results indicate that as long as the geometry of the molecule in the triplet state is not very different from that of the singlet state, only a very small rotational dependence will show up in the singlet state radiationless lifetime, in spite of the fact that the overall dissociation constitutes a large geometry change.

A model for isotope separation via molecular multiphoton photodissociation

Multiphoton photofragmentation of an “isolated” molecule on the lowest potential surface is considered in terms of a truncated anharmonic oscillator driven by a pulsed, intense laser field. Intramolecular vibrational relaxation and indirect photodissociation are accounted for by assignment of decay widths to the appropriate levels. The effective-hamiltonian formalism is applied to derive explicit expressions for the photofragmentation yield and for the isotopic separation factor.