Diffuse Vibrational Structure Research Articles

Ultraviolet spectroscopy of pressurized and supercritical carbon dioxide

Carbon dioxide (CO2) is prevalent in planetary atmospheres and sees use in a variety of industrial applications. Despite its ubiquitous nature, its photochemistry remains poorly understood. In this work we explore the density dependence of pressurized and supercritical CO2 electronic absorption spectra by vacuum ultraviolet spectroscopy over the wavelength range 1455-2000 Å. We show that the lowest absorption band transition energy is unaffected by a density increase up to and beyond the thermodynamic critical point (137 bar, 308 K). However, the diffuse vibrational structure inherent to the spectrum gradually decreases in magnitude. This effect cannot be explained solely by collisional broadening and/or dimerization. We suggest that at high densities close proximity of neighboring CO2 molecules with a variety of orientations perturbs the multiple monomer electronic state potential energy surfaces, facilitating coupling between binding and dissociative states. We estimate a critical radius of ~4.1 Å necessary to cause such perturbations.

Decomposing the First Absorption Band of OCS Using Photofragment Excitation Spectroscopy.

Photofragment excitation spectra of carbonyl sulfide (OCS) have been recorded from 212-260 nm by state-selectively probing either electronically excited S((1)D) or ground state S((3)P) photolysis products via 2 + 1 resonance-enhanced multiphoton ionization. Probing the major S((1)D) product results in a broad, unstructured action spectrum that reproduces the overall shape of the first absorption band. In contrast, spectra obtained probing S((3)P) products display prominent resonances superimposed on a broad continuum; the resonances correspond to the diffuse vibrational structure observed in the conventional absorption spectrum. The vibrational structure is assigned to four progressions, each dominated by the C-S stretch, ν1, following direct excitation to quasi-bound singlet and triplet states. The S((3)PJ) products are formed with a near-statistical population distribution over the J = 2, 1, and 0 spin-orbit levels across the wavelength range investigated. Although a minor contributor to the S atom yield near the peak of the absorption cross section, the relative yield of S((3)P) increases significantly at longer wavelengths. The experimental measurements validate recent theoretical work characterizing the electronic states responsible for the first absorption band by Schmidt and co-workers.

Full-dimensional quantum molecular dynamics calculations of the rovibrationally mediated X 1A′ → 2 1A′′ transition of nitrous oxide

A full dimensional time-dependent quantum wavepacket approach is used to study the photodissociation dynamics of nitrous oxide for the X 1A′ → 2 1A′′ bound–bound transition based on new highly accurate potential energy and transition dipole moment surfaces. The computed 2 1A′′ absorption spectra at room temperature are characterized by sharp vibrational structures that contribute slightly to the diffuse vibrational structures around the maximum peak at 180 nm of the first ultraviolet absorption band (from the contribution of 2 1A′, 1 1A′′, and 2 1A′′ states) of N2O. Transitions from different initial rovibrational states reveal that the sharp structures arise mainly from N2O bending vibrations, whereas, at higher temperatures, the N2O and NNO stretching vibrations are responsible for enhancing the intensity of the structures. At absorption wavelengths 166 nm and 179 nm, vibrational quantum state distributions of N2 product fragments decrease monotonically with increasing vibrational quantum number v = 0, 1, 2. At 166 nm, rotational quantum state distributions of N2 at fixed v = 0 and v = 1 display multimodal profiles with maximum peaks at j = 77 and j = 75, respectively, whereas, the distributions at the 179 nm absorption wavelength display bimodal profiles with maximum peaks at j = 73 and j = 71, respectively. Accordingly, the presence of rotationally hot N2 from previous experimental and theoretical works in the first band strongly implies a significant influence of the 2 1A′′ state in determining the final dissociation pathway of N2 + O. © 2016 Wiley Periodicals, Inc.

Communication: Photodissociation of N2O—Frustrated NN bond breaking causes diffuse vibrational structures

The photodissociation of N(2)O is studied by wave packet calculations using a global three-dimensional potential energy surface for the first excited (1)A(') state. It is shown that the weak vibrational structures of the absorption cross section are caused by large-amplitude NN stretch motion, combined with strong excitation of the bend as well as the O-NN stretch. Weakening of the NN bond toward the N+NO channel is the necessary prerequisite.

The photodissociation of NO 2 in the second absorption band: Ab initio and quantum dynamics calculations

The photodissociation of NO 2 in the second absorption band (217 nm) is investigated by electronic structure as well as quantum mechanical and classical dynamics calculations. Potential energies are calculated for the five lowest 2A′ states using the internally contracted multi-reference configuration interaction method and the standard correlation consistent polarized quadruple zeta basis set. The two N–O bond lengths and the O–N–O bond angle are varied over wide ranges. Our calculations unambiguously verify that the second absorption band is due to excitation of state 3 2A′ (2 2B 2 in C 2 v ), which is the only state with non-vanishing transition dipole moment in this energy range. State 3 2A′ correlates diabatically with O( 1D) + NO( 2Π), but it connects adiabatically with the ground-state product channel O( 3P) + NO( 2Π). In the present study, we construct a three-dimensional potential energy surface (PES) for only one adiabatic state, 3 2A′, and primarily focus on the absorption cross-section due to excitation of this state. The general form of the absorption spectrum and the progression of diffuse vibrational structures are well reproduced by the quantum mechanical calculations. The vibrational and rotational state distributions of NO in the O( 3P) + NO( 2Π) channel are also determined. The predicted internal energy distributions are relatively narrow over the entire range of the absorption spectrum – in quantitative agreement with the experimental result for dissociation at 226 nm, but in severe qualitative disagreement with measurements near 202 nm.

New theoretical investigations of the photodissociation of ozone in the Hartley, Huggins, Chappuis, and Wulf bands

We review recent theoretical studies of the photodissociation of ozone in the wavelength region from 200 nm to 1100 nm comprising four major absorption bands: Hartley and Huggins (near ultraviolet), Chappuis (visible), and Wulf (near infrared). The quantum mechanical dynamics calculations use global potential energy surfaces obtained from new high-level electronic structure calculations. Altogether nine electronic states are taken into account in the theoretical descriptions: four 1A', two 1A'', one 3A' and two 3A'' states. Of particular interest is the analysis of diffuse vibrational structures, which are prominent in all absorption bands, and their dynamical origin and assignment. Another focus is the effect of non-adiabatic coupling on lifetimes in the excited states and on the population of the specific electronic product channels.

UV absorption cross sections for SO3

Room temperature gas phase absorption cross sections for SO3 have been measured over the wavelength range 195 to 330 nm using a diode array spectrometer. The SO3 spectrum is continuous with weak diffuse vibrational band structure in the 225 to 295 nm region. Atmospheric photolysis rate calculations show that photolysis of SO3 in the 190 to 230 nm actinic window is important in determining the sulfur partitioning in the upper stratosphere.

Photodissociation of ozone in the Chappuis band. II. Time-dependent wave-packet calculations and interpretation of diffuse vibrational structures

We present time-dependent wave-packet calculations describing the photodissociation of ozone in the Chappuis band, which evolves in the two lowest states of A″1 symmetry. The calculations are performed in the diabatic representation and include the coupling between the two relevant states. All three nuclear degrees of freedom are taken into account. The two potential-energy surfaces, the coupling potential, and the two transition-dipole-moment functions with the electronic ground state have been calculated previously by ab initio methods [Woywod et al., J. Chem. Phys. 107, 7282 (1997)]. The coupling between the two diabatic states is exceedingly strong, resulting in very fast dissociation into O+O2 on the time scale of only one symmetric stretch period. A small portion of the initially created wave packet is temporarily trapped leading to three tiny recurrences, which reflect basically symmetric stretch motion plus some amount of bending motion. The experimentally observed diffuse vibrational structures superimposed to the broad absorption spectrum are satisfactorily reproduced and discussed both in the time-independent and the time-dependent picture of spectroscopy. In view of the very short lifetime in the excited states, the correspondingly large widths of the diffuse structures, and the participation of all three vibrational modes we conclude that an unique assignment in terms of three quantum numbers is not possible. The main structures are due to symmetric stretch excitation, but bending and even asymmetric stretch motion are also involved. A one-state model, in which only the upper (bound) adiabatic potential is employed, provides a qualitatively correct explanation of the absorption spectrum and the diffuse structures.

Phase Recovery from the Raman Excitation Profile, Time Domain Information and Transform Theory

The phase recovery from the vibrational Raman excitation profile (REP), which contains only the modulus of the Raman amplitude, is discussed for the general situation where the Raman amplitude, with excitation energy extended in the complex plane, may have zeros in the right-half plane. The focus is on the dispersion method, with all results derived by contour integration. The new results for phase recovery, however, apply to both the dispersion and maximum entropy methods. An iterative procedure, with rapid convergence, is presented to overcome the experimental REP data being given in a limited energy range. The forward transform from the electronic absorption spectrum (ABS) to the REP and the inverse transform from the REP to the ABS are presented in a unified manner. The ubiquitous Hilbert transform is shown to be readily evaluated by the fast Fourier transform algorithm. Calculations are presented for β-carotene, a two-mode harmonic model with diffuse vibrational structure, azulene and iodobenzene to illustrate the theory. © 1997 John Wiley & Sons, Ltd.

A quantum mechanical, time-dependent wave packet interpretation of the diffuse structures in the S→S1 absorption spectrum of FNO: Coexistence of direct and indirect dissociation

We have investigated the photodissociation of FNO in the first absorption band (S0→S1) by a two-dimensional wave packet study based on an ab initio potential energy surface. The quantum chemical calculations were performed in the multiconfiguration self-consistent field (MCSCF) approach including the N–O and the F–NO bond distances with the FNO bond angle being fixed. The most striking feature of the time-dependent dynamical analysis is a bifurcation of the wave packet near the Franck–Condon point: while one part of the wave packet leaves the inner region of the potential energy surface very rapidly, a second part remains trapped for several periods in an extremely shallow well at short F–NO distances. The direct part leads to a broad background in the absorption spectrum while the trapped portion of the wave packet gives rise to relatively narrow resonances, i.e., well resolved diffuse vibrational structures. The bandwidth decreases with the degree of internal excitation. The calculated spectrum agrees well with the measured one.

Diffuse structures and periodic orbits in the photodissociation of small polyatomic molecules

The relation of diffuse vibrational structures in UV-absorption spectra of (small) polyatomic molecules and internal vibrational motion in excited electronic states are investigated. The method of choice is the propagation of time-dependent wavepackets with the autocorrelation function serving as the link between the energy dependence of the spectrum and the time dependence of the molecular motion in the excited electronic state. For the purpose of this paper we characterize diffuse structures as very short-lived resonances with ‘lifetimes’ of the order of at most one internal vibrational period. In particular, we study a model system for the photodissociation of symmetric triatomic molecules ABA such as H2O, CO2 and O3, the photodissociation of H2O in the second continuum, and the fragmentation of H2S. The existence of unstable periodic orbits and their influence on the dissociation dynamics is especially elucidated. In the case of H2S we demonstrate that the diffuse absorption structures are caused by symmetric stretch motion in a binding state which is strongly coupled to a dissociative state. Diffuse structures can be regarded as very broad resonances in excited electronic states. They manifest transition-state spectroscopy in the original sense of the word.

Photodissociation dynamics of jet-cooled ClNO on S1(1 1A″): An experimental study

We report measurements of photofragment yield (PHOFRY) spectra and NO E, V, R distributions following dissociation of jet-cooled ClNO on the S1(1 1A″) electronic surface. The dissociative S1(1 1A″)←S0(1 1A′) transition shows diffuse vibrational structure with a progression in ν1, the NO stretch. The absorption and PHOFRY spectra consist of two bands, corresponding to excitations into S1(000) and S1(100), whose widths are 1300±100 and 1000±70 cm−1, respectively. The relative partial absorption cross sections are S1(000):S1(100)=2.3:1.0. The narrowing of the absorption bands with increasing ν1 quanta is a consequence of the mismatch between ν1 and the free NO vibrational frequency. Dissociations on S1(000) and S1(100) yield NO in v″=0 and 1, respectively. The NO(X2∏) rotational distributions in v″=0 and 1 are inverted, peaking at J″∼30.5 with widths of 10±1 J″, and they do not vary significantly when the photolysis laser is scanned across the absorption band. The evolution of NO vibrational and rotational excitations appear to be largely uncoupled. In NO v″=0 and 1, the upper spin–orbit state 2∏3/2 is more populated than the lower state 2∏1/2. For both v″=0 and 1, the Λ-doublet ∏(A″) component of NO(2∏1/2) is more populated than the ∏(A′) component by a ratio of ∼3:1, as expected for excitation to a π* orbital of a″ symmetry, but this propensity is much lower for NO(2∏3/2), possibly due to perturbations with another surface. The absorption spectra and NO V, R distributions are in good agreement with recent dynamical calculations on a three-dimensional (3-D) potential-energy surface (PES) calculated ab initio. The vibrational distribution appears to be determined near the Franck–Condon (FC) region, while final-state interactions affect the rotational distributions at larger Cl–NO separations.

The influence of excited-state vibrations on fragment state distributions: The photodissociation of NOCl on T1(1 3A″)

NO V,R distributions are reported following photodissociation of jet-cooled NOCl from selected vibrational levels of the T1(1 3A″) state. By varying the photolysis wavelength while monitoring selected rovibrational levels of NO, a photofragment yield spectrum showing the same diffuse vibrational structure as the absorption spectrum is obtained. NO rotational distributions are obtained at photolysis wavelengths corresponding to peaks in the absorption spectrum. We find that the NO stretching vibrations of T1 evolve adiabatically into NO vibrational excitations [i.e., excitation of v1 quanta of the T1 NO stretch yields predominantly NO(v″=v1) ]. The NO rotational distributions depend only on the number of T1 bending quanta, v3. The shapes of the distributions reflect the number of nodes in the bending wave functions, and similar rotational distributions are obtained following excitation of vibronic levels with equal number of ν3 quanta, but different number of ν1 quanta (v1=0–2). We also find that the excited NO(2Π3/2) state is much more populated than the lower NO(2Π1/2) state, and the widths of the absorption features increase with increasing number of ν3 quanta, but decrease with increasing ν1 excitation. The results are explained in terms of a model in which the bending and NO stretch motions in the excited state are largely uncoupled. In order to explain the multimodal rotational distributions, we calculate separately the components of the NO rotational excitation that derive from the angular momentum inherent in the bending wave function and the angular anisotropy in the potential-energy surface. We find, using the momentum representation of the harmonic oscillator, that the rotational distributions map the bending wave functions and exhibit minima for v3>0. In addition, a unidirectional torque generated by the angular anisotropy in the potential causes shifts in the distributions calculated by the pure Franck–Condon model that are bending level dependent. The observation that the absorption linewidth decreases with increasing NO stretch excitation in the parent T1 state may be a manifestation of the energy mismatch between the frequency of the NO stretch in NOCl and free NO; the increased mismatch with increasing stretch quantum number results in slower dissociation.

Unstable periodic orbits, recurrences, and diffuse vibrational structures in the photodissociation of water near 128 nm

The photodissociation of H2O in the second absorption band (X̃→B̃) is investigated in a completely time-dependent approach. The Schrödinger equation is solved by a time-dependent close-coupling method expanding the two-dimensional wave packet in terms of free rotor states. The vibrational degree of freedom of the OH fragment is fixed and only motion on the B̃-state potential-energy surface is considered. The calculated absorption spectrum exhibits a long progression of diffuse structures, ΔE∼0.1 eV, in very good agreement with the experimental spectrum. The structure is readily explained in terms of a recurrence of the autocorrelation function after about 40 fs. The recurrence, in turn, is attributed to special indirect trajectories which on the average perform one oscillation within the deep potential well before they dissociate into products H+OH. These trajections are ‘‘guided’’ by so-called unstable periodic orbits which persist to energies high above the H+OH(2 Σ) threshold. The existence of unstable periodic orbits leading to a recurrence of the autocorrelation function gives, for the first time, a consistent explanation of the diffuse structure in the absorption spectrum of H2 O in the second band.

Diffuse vibrational structures in photoabsorption spectra: A comparison of CH3ONO and CH3SNO using two-dimensional a b i n i t i o potential energy surfaces

We investigated the photodissociation of methyl nitrite (CH3 ONO) and methyl thionitrite (CH3 SNO) within the first absorption band (S1 ←S0 ). The calculations were based on a two-dimensional model including the O–NO/S–NO and N=O bond distances as active coordinates. The S1 -potential energy surfaces were calculated with quantum chemical methods and the dynamical calculations were performed exactly within the time-independent approach. The main emphasis is on the origin of diffuse vibrational structure in the photoabsorption spectrum of both molecules. A low potential barrier of 0.086 eV along the O–NO dissociation coordinate in CH3 ONO prevents immediate dissociation and leads to an initial state dependent lifetime for the excited complex of 100–250 fs corresponding to 3–8 NO vibrational periods. CH3 ONO decays nonadiabatically via vibrational predissociation. The absorption spectrum of CH3 ONO is dominated by narrow Feshbach-like scattering resonances which can be characterized by two quantum numbers, m and n*: m=0 and 1 specifies the quanta of excitation in the O–NO bond and n*=0,1,2,... specifies the excited vibrational level of the N=O bond. The potential barrier is absent in CH3 SNO and the dissociation is direct on the time scale of about 10 fs corresponding to only one third of a NO vibrational period. Nevertheless, the absorption spectrum exhibits diffuse vibrational structures. The shape of the individual absorption peaks is determined by the classical Franck–Condon reflection principle. The dissociation of CH3 SNO is primarily adiabatic which leads to a pronounced energy dependence of the final NO vibrational state distribution. The diffuse structures originate in both cases from excitation of the NO stretching vibration. In order to make contact with time-dependent theory we calculated the autocorrelation function of the time-dependent wave function by inverse Fourier transformation of the energy-dependent spectra. The agreement with available experimental data for both molecules is quite satisfactory. This includes the energy spacing of the vibrational structure, the overall shape of the absorption spectrum, and the lifetime of the excited complex.

Photodissociation dynamics of water in the second absorption band. II. A b i n i t i o calculation of the absorption spectra for H2O and D2O and dynamical interpretation of ‘‘diffuse vibrational’’ structures

We calculated the absorption spectra of H2O and D2O in the second absorption band around 128 nm using a two-dimensional ab initio potential energy surface for the B̃(1A1) electronic state. Nonadiabatic coupling to the lower states à and X̃ and the vibrational degree of freedom of the OH fragment are completely neglected. Despite these limitations the agreement with the measured spectra is very satisfactory. The overall shape, the width, and the energetical position of the maximum are well described. Most important, however, is the reproduction of the diffuse vibrational structures superimposed on the broad background. It is demonstrated that this structure is not caused by pure bending-excitation in the B̃ state with associated bending quantum numbers ν′2=1,2,3,... as originally assumed. Because the equilibrium HOH bending angle and the equilibrium H–OH distance are very different in the ground and in the excited state, the main part of the spectrum and especially the diffuse structures occur at high energies within the continuum of the B̃ state potential energy surface. Within the time-dependent approach, based on the autocorrelation function and simple classical trajectories, it is shown that the diffuse structures originate from the temporary excitation of a large amplitude bending and stretching oscillation embedded in the continuum (short lived quasiperiodic orbits). The vibrational period of this mode is approximately 40 fs and the lifetime of the trapped trajectories is on the average one vibrational period.

ChemInform Abstract: ANOMALOUS V1 ← N VIBRATIONAL BAND INTENSITIES IN THE LOW ENERGY ELECTRON IMPACT SPECTRA OF TRANS‐DIENES

A detailed examination of the extremely broad and diffuse vibrational structure of the V1←N allowed valence transition in trans‐1,3‐butadiene, three related trans‐dienes (cis‐1,3‐pentadiene, trans, trans‐2,4‐hexadiene, and 2,3‐dimethyl‐1,3‐butadiene), and the cis‐diene 1,3‐cyclohexadiene has shown that the effect observed previously by Doering, in which the lower vibrational bands of the transition are enhanced in intensity relative to the higher bands at low incident electron energies, occurs in all the trans‐dienes studied. This result shows that the effect cannot be due to an underlying, forbidden electronic transition. It appears that in these trans‐dienes, the configurational composition of the V1 ’’state’’ depends on the geometry of the molecule and that the configurational components of the excited state that are important in the vertical transition have different electron impact excitation cross sections than those that are important in the adiabatic transition. These results suggest that electron...

Anomalous V1←N vibrational band intensities in the low energy electron impact spectra of t r a n s-dienes

A detailed examination of the extremely broad and diffuse vibrational structure of the V1←N allowed valence transition in trans-1,3-butadiene, three related trans-dienes (cis-1,3-pentadiene, trans, trans-2,4-hexadiene, and 2,3-dimethyl-1,3-butadiene), and the cis-diene 1,3-cyclohexadiene has shown that the effect observed previously by Doering, in which the lower vibrational bands of the transition are enhanced in intensity relative to the higher bands at low incident electron energies, occurs in all the trans-dienes studied. This result shows that the effect cannot be due to an underlying, forbidden electronic transition. It appears that in these trans-dienes, the configurational composition of the V1 ’’state’’ depends on the geometry of the molecule and that the configurational components of the excited state that are important in the vertical transition have different electron impact excitation cross sections than those that are important in the adiabatic transition. These results suggest that electron energy loss spectroscopy is a unique tool for probing the configurational composition of certain highly distorted excited states.

Simple theory of diffuse vibrational structure in continuous uv spectra of polyatomic molecules. I. Collinear photodissociation of symmetric triatomics

A theory for the direct photodissociation of polyatomic molecules is presented, and it is shown that uv absorption spectra into simply dissociative upper electronic states can have considerable vibrational structure. This provides a simple interpretation of many of the ’’diffuse vibrational bands superimposed on continua’’ often seen in polyatomic spectra.

Vacuum ultraviolet spectra of osmium tetroxide and ruthenium tetroxide

The gas phase absorption spectra of the compounds OsO4 and RuO4 were measured between 2500 and 1050 Å with a Baird 1−m vacuum monochrometer of 1.0 Å bandwidth. Both tetroxides show many electronic absorption bands in this region with diffuse vibrational structure. No spectra of OsO4 or RuO4 have been previously reported below 2200 Å. Absorption maxima for OsO4, which were not previously reported, occur at 48 140, 63 620, 73 960, 85 620, 86 920, 89 160, 90 800, and 91 940 cm−1. The spectrum of RuO4 showed band maxima at 50 660, 64 620, 72 700, 76 420, 79 040, 83 100, 88 860, and 92 080 cm−1. The general appearance and relative strengths of various absorption bands in the spectra of these molecules were similar, with the energy of apparently corresponding transitions shifted to the red in RuO4 by approximately 10 000 cm−1. Most of the strong electronic transitions have been tentatively assigned on the basis of extended Hückel type calculations. The results of these calculations correlated reasonably well with the spectral data obtained. Bands are present in the spectra of both molecules which may be the first members of Rydberg series.