Rovibronic States Research Articles

A study of the S 1 6 1 ( 1A″ 2) vibronically excited state of sym-triazine by high-resolution UV laser spectroscopy

We have measured the laser induced fluorescence spectrum of the S 1 6 1 ( 1A″ 2)←S 0( 1A′ 1) vibronic transition in sym-triazine at 317 nm. An instrumental line width of 12 MHz was achieved by the use of a molecular beam in combination with a cw single frequency laser. In this way rotationally resolved spectra were obtained. Furthermore, we have measured the lifetime of some selected ensembles of rovibronic states with a narrow band pulsed laser system. It is shown that the excited S 1 6 1 (A″ 2) vibronic state is strongly perturbed by a singlet-triplet interaction. This interaction results in additional lines in the high-resolution spectrum. A nearly complete rotational assignment is given for the transitions to the upper state with rotational quantum number J up to J = 4 and the rotational constants are derived. No evidence is found for a Jahn-Teller distortion of the rotational states in the S 1 6 1 (A″ 2) vibronic state. With some assumptions the singlet-triplet coupling matrix elements were deduced as well as the energy separation between the singlet state and the perturbing triplet state. A singlet-triplet gap of 4800 ± 600 cm −1 is estimated.

CARS spectroscopy of the NaH2 collision complex: the nature of the Na(32 P)H2 exciplex ? ab initio calculations and experimental results

CARS (Coherent Anti-Stokes Raman Scattering) has been used to analyze the rovibronic state distribution of H2 after collision with Na(32 P). New lines, which do not correspond to H2 lines are observed in the CARS spectrum. The experiments point to the formation of a complex of Na(32 P)H2 inA 2 B 2 symmetry. Ab initio calculations of theA 2 B 2 potential were performed. On this surface the vibrational spectra of the exciplex are evaluated. The observed lines can be attributed to vibrational transitions in the complex, in which combinational modes are involved. The connection of experimental and theoretical results indicates that a collisionally stabilized exciplex molecule is formed during the quenching process.

Rotationally resolved fluorescence-dip and ion-dip spectra of single rovibronic states of benzene

We report fluorescence-dip as well as ion-dip spectra of single rovibronic one-photon states of benzene with a linewidth as narrow as 0.14 cm−1. The selective excitation of the rovibronic states was achieved through the combination of a frequency-doubled pulsed amplified cw dye laser (ΔνUV ≈ 100 MHz) and a collimated molecular beam. The detailed analysis of the dip spectra shows that the observed spectral features correspond to single rovibronic transitions if suitable states are excited. From the spectra, precise harmonic frequencies and anharmonic constants for the S0 state are determined. A hitherto unknown Darling–Dennison resonance of the overtones of ν1 with the 52 state is found.

Rotational effects in the continuous vacuum-ultraviolet fluorescence spectrum of H2 associated with spontaneous dissociation.

The effects of rotational-vibrational interaction, and of rotational coupling between the {ital B} 2{ital p} {sup 1}{Sigma}{sub {ital u}}{sup +} and {ital C} 2{ital p} {sup 1}{Pi}{sub {ital u}} electronic states, on the {ital B}--{ital X} fluorescence continuum of H{sub 2} associated with spontaneous dissociation were investigated spectroscopically using monochromatized synchrotron radiation for selective excitation of rovibronic states. The rotational shifts and perturbations in the modulated continua intensities observed agree well with close-coupling and Jeffreys-Wentzel-Kromers-Brillouin (JWKB) calculations also performed, and are explained in terms of nonadiabatic coupling effects and rainbow-interference structures. A straightforward experimental technique is described to detect mixed electronic states.

Back to the roots of ‘‘channel three’’: Rotationally resolved spectra of the 6113 band of C6H6

Rotationally resolved fluorescence excitation and resonance enhanced multiphoton ionization (MPI) spectra of the 610130 one-photon band of benzene at the onset of ‘‘channel three’’ are reported. The fluorescence decay is monitored after rotationally selected excitation and a large variation of the nonradiative decay time (<1 to 6.8 ns) is found for the different rotational states at the vibrational excess energy of 3287 cm−1 in S1. The rotational structure of the fluorescence excitation spectrum and the MPI spectrum measured with delayed laser pulses differ considerably. All observed lines of the MPI spectrum were assigned and the rotational line structure can only be understood with a model which incorporates interference between different decay channels. Due to this interference, particular rotational states decay fairly slowly and give rise to lines in the spectrum while states with neighboring rotational quantum numbers decay rapidly and are therefore not found in the spectrum. The previously reported drastic increase of the electronic, nonradiative decay of benzene in this region of excess energy, which led to the postulation of ‘‘channel three,’’ cannot be confirmed. Instead, the optically excited rovibronic states are thought to be coupled to background states within S1 which are themselves broadened due to strong coupling to the highly excited S0 electronic state rather than due to an unknown (‘‘channel three’’) or isomerization process.

Rotational effects in the VUV continuum of H2

The effects of rotation-vibration interaction and of rotational coupling between the B 2p 1Σu+ and C 2p 1Πu electronic states on the B-X continuum of H2 were investigated spectroscopically using monochromatized synchrotron radiation for selective excitation of rovibronic states. The perturbed continuum intensities observed for the first time agree well with close-coupling calculations also performed.

An alternative mechanism for spin-forbidden photo-ionization of diatomic molecules and its rotation–electronic selection rules

The spin-forbidden photo-ionization of diatomic molecules is proposed. Spin orbit interaction is invoked resulting in the correction and mixing of the wave functions of different multiplicities. The rotation–electronic selection rules given, but not proven, by Dixit and McKoy for Hund's case a based on the conventional mechanism of electric dipole transition (see Chem. Phys. Lett. 128, 49 (1986) are rederived and expressed in a different format. This new format permits the generalization of the selection rules to other photo-ionization transitions caused by the magnetic dipole, the electric quadrupole, and the two- and three-photon operators. These selection rules, which are for transitions from one specific rotational level of a given Kronig reflection symmetry to another, will help understand rotational branching and the dynamics of interaction in the excited state. They will also help in the selective preparation of well-defined rovibronic states in resonant-enhanced multi-photon ionization processes.

Dynamic Behavior of Individual Rovibronic States in S1 Benzene

AbstractPulsed Doppler‐free two‐photon excitation of single rotational states of the 141 vibronic state of benzene with a resolution of 70 MHz is employed to investigate the influence of molecular rotation on the decay behavior of these states. The analysis of the completely resolved (Δv = 15 MHz) spectrum of the 1410 band, measured with a CW laser, makes it possible to specify whether individual states are perturbed by other (“dark”) S1 states and allows the precise determination of the degree of admixture of these background states. The vibrational identity of the background states is known from emission spectra of single rovibronic states. Unperturbed rovibronic states were investigated up to J'K' = 6761, and a pure exponential decay was found which does not depend on molecular rotation in accord with a nonradiative decay of S1 benzene in the statistical limit. The decay of perturbed states, which was also found to be purely exponential, depends strongly on the degree of admixture of the background state and on its vibrational identity. Values of the decay time of two identified “dark” zero‐order S1 vibrational states, which cannot be directly excited by optical transitions, are determined by evaluation of the decay behavior of the perturbed states.

Emission spectra from single rovibronic quantum states in S1 benzene after Doppler-free two-photon excitation

Dispersed emission from single rovibronic quantum states in S1 benzene is measured after Doppler-free two-photon excitation under low pressure conditions (0.3 Torr). This was made possible by a long-term stabilization of the single-mode dye laser yielding a stability of better than 1 MHz/h. The emission spectra of unperturbed rotational levels in the 141 and the 14111 vibronic states reveal a great number of detailed results on Duschinsky rotation and long-range Fermi resonances in the electronic ground state. By contrast, it is seen that the emission spectra from perturbed rovibronic states are contaminated by additional bands. The analysis of these bands leads in most cases to an identification of the coupled dark background state and the responsible rotation–vibration coupling process (H42 resonances). The emission spectra clearly demonstrate that even for a density of states of 60 1/cm−1, coupling in S1 benzene is still selective and far from the statistical limit. It is further demonstrated that the dark and the light states are more efficiently mixed by short-range couplings with coupling matrix elements of some GHz than by long-range Fermi resonances.

Analysis and deconvolution of some J′≠0 rovibronic transitions in the high resolution S1←S fluorescence excitation spectrum of pyrazine

Fluorescence excitation spectra are reported for several J′≠0 rotational members of the 000 band of the S1(1B3u)←S0(1A1g) electronic transition of pyrazine at a resolution of about 10 MHz. The transitions studied, namely R(0)–R(2) and P(2)–P(4), are each split into a large number of sharp lines ascribed, as in the case of the previously studied P(1) (J′=0) transition, to coupling with the lowest triplet state T1(3B3u). Despite this complexity, we show in this paper that it is possible to separate the lines into clusters of transitions that terminate in the same K′ component of the electronically excited, mixed S1–T1 state. This demonstrates that K′ is a good quantum number, at least at low J′ in the zero-order S1 state. From this analysis, we determine the rotational constants of the S0 and S1 states. We also determine: (i) the relative cluster intensities; (ii) the coupled T1 level densities; and (iii) by using standard deconvolution techniques, the S1–T1 coupling matrix elements, each as a function of J′,K′. Cluster intensities decrease with increasing J′, but K′=0 clusters are significantly less intense than K′≠0 clusters in the fluorescence excitation spectra. Observed triplet level densities in each cluster exceed by an order of magnitude the calculated density of rovibronic states if selection rules appropriate to the D2h point group are taken into account. Neither the observed level densities nor the coupling matrix elements (which vary from less than 5 MHz to more than 500 MHz) show a clear-cut systematic dependence on J′ or K′, although K′=0 levels appear to be more strongly coupled than K′≠0 levels. Possible explanations for these results and their implications for intersystem crossing dynamics in the isolated molecule are discussed.

Observation of rotational predissociation in 20NeH +, 22NeH +, 20NeD +, 40ArH + and D 2+ formed from various precursors

Translational energy spectroscopy has been used to investigate the unimolecular dissociation, at keV translational energies, of NeH + → H +, ArH + → H + and D + 2 → D +. For D + 2 in its ground electronic state ( X 2Σ + g), fragmentation is known to proceed solely by the mechanism of rotational predissociation. This study indicates the same mechanism to account for the unimolecular fragmentation of NeH + and possibly for ArH +. For D + 2 five quasibound states have been previously detected using a mass spectrometric technique. In this experiment, D + 2 was formed from a number of deuterated organic molecules, by dissociative electron impact ionization CD 4 was found to produce the largest D + 2 signal, where a total of at least ten and possibly more quasibound states have been identified. The energy releases have been measured for all the observed predissociating states. For D + 2, where theoretical calculations exist, the initial relative populations have been determined and found to lie in a range differing by no more than a factor of four. Populations of rovibronic states were found to depend on the precursor used to form D + 2 and were estimated from the translational energy spectra. For 20NeH +, 22NeH + and 20NeD +, five, three and five predissociating states were observed, respectively, whilst for 40ArH + the very low signal to noise ratio of the H + product allowed only a tentative assignment of the fragmentation mechanism and estimation of a single energy release.

Hyperfine structure near the 13–1 band head in the B–X transition of ^127I_2

Hyperfine structure of the rovibronic transitions R(0)–R(10) and P(1)–P(5) in the 13–1 band belonging to the B–X system of 127I2 was studied by rectangular-crossing molecular-beam laser spectroscopy. The full width at half-maximum of the observed lines was 5 MHz, which corresponds to a resolving power of 108. Not only allowed transitions ΔF = ΔJ but forbidden transitions ΔF = 0 and ΔF = −ΔJ were identified, and absolute hyperfine coupling constants of the rovibronic states (J = 0–10) in the B(v′ = 13)−X(v″ = 1) transition were determined. In addition, the relative line strength of each hyperfine transition was compared with the theoretical value.

Collisional perturbation of rovibronically selected sulfur dioxide(~A1A2) in a supersonic jet

The fluorescence spectrum and its intensity decay of SO/sub 2/(anti A /sup 1/A/sub 2/) in single rovibronic levels of the E band have been observed in a supersonic jet to discuss the collisional transfer between rovibronic states. It was found that the undispersed fluorescence of SO/sub 2/(anti A /sup 1/A/sub 2/) decayed in a nonexponential manner, while the dispersed fluorescence of the transition to the (1,0,0)'' state of anti X /sup 1/A/sub 1/ showed almost a single-exponential decay. The fluorescence intensity of the transition to the anti X /sup 1/A/sub 1/)'' state from certain rovibronic levels increased after the laser irradiation and reached a peak before it decayed. These experimental facts indicated that SO/sub 2/(anti A /sup 1/, A/sub 2/) in the initially prepared rovibronic level makes a transition to a state having a longer radiative lifetime, which is induced by low-energy collisions in a supersonic jet. The final state is probably a perturbed vibronic state of anti A /sup 1/A/sub 2/. The day process was analyzed kinetically to obtain the relevant fluorescence lifetimes of 6-13 ..mu..s and collisional cross sections on the order of several hundred square angstroms.

Simultaneous interaction of Coriolis and anharmonic coupling in the intramolecular vibrational redistribution of polyatomic molecules

We show theoretically that the simultaneous interaction of Coriolis and anharmonic coupling contributes significantly to the intramolecular vibrational redistribution (IVR) of rovibronic states excited by a pulse laser with a narrow bandwidth. The mechanism of the IVR of S 1 benzene in the region of the third channel is briefly discussed, based on the derived rate expression.

Dissociation of H 2+ prepared in different rovibronic states by keV collisions with rare gases

Translational energy spectra for H + ions produced by the keV collision-induced dissociation of H 2 + on rare gases were recorded on a reversed-geometry mass spectrometer. Results were obtained for various vibrational populations of the precursor H 2 + (1 sσ g ) state formed by electron impact. Experimental kinetic energy distributions are compared with a 1 sσ g → 2 pσ u Franck-Condon calculation. A new dissociative mechanism involving vibrational excitation of H 2 + (1 sσ g ) to quasi-bound states, and subsequent predissociation, is proposed to explain the formation of H + fragments with centre of mass kinetic energy release less than 0.3 eV.

Theory of coherent polarization anisotropy in time-resolved two-photon ionization of isolated molecules. Effects of Coriolis couplings

Effects of vibration–rotation (Coriolis) couplings on the coherent polarization anisotropy are theoretically studied in a time-resolved two-photon ionization of a symmetric top molecule. This polarization anisotropy originates from a coherent excitation of the resonant rovibronic molecular eigenstates (rovibronic coherence) whose zeroth order states are mixed through the Coriolis coupling. Expressions for the time-dependent degree of polarization after the coherent excitation of the rovibronic states produced by the Coriolis coupling are derived as a function of the delay time in the pump–probe two-photon ionization. Model calculations of the time-dependent degree of polarization as well as the probabilities of the two-photon ionization are performed to demonstrate the Coriolis coupling effects in the low excess energy regions of the resonant intermediate state. It is shown that oscillatory behaviors in the time-dependent degree of polarization should be observed as a result of the creation of the rovibronic coherence. It is demonstrated that oscillations of the degree of polarization involve both contribution of the purely rotational J-coherence and that of the rovibronic coherence in the resonant manifold when the rotational branches are coherently excited and the characteristic rotation–vibration interaction energy is larger than a typical free rotational energy under jet-cooled condition.

Model studies for rovibronic IVR with applications to benzene

Intramolecular vibrational energy redistribution (IVR) is treated. Nearly isoenergetic rigid-rotor harmonic-oscillator states are rovibronically coupled by the interplay of anharmonic and Coriolis forces. The theory predicts linewidth distributions of rovibronic states and explains the selective supression of rotational spectral lines. Applications to sub-Doppler spectra of the 1410120 band of benzene allow to extract values for effective intramolecular interactions. These results are supported by direct calculations based on the force field.

Three-dimensional analytical quantum mechanical theory for triatomic photodissociation: Role of angle dependent dissociative surfaces on rotational and angular distributions in the rotational infinite order sudden limit

An analytical quantum mechanical model is developed for calculating fragment energy distributions in photodissociation of linear triatomic molecules when the repulsive potential energy surface is anisotropic. The translational continuum function is taken to be given by the infinite order sudden approximation, but an equivalent adiabatic interpretation leads to a natural choice for the diatomic angular momentum j and for the retention of asymptotic rotational energy differences. Normal coordinates are used for the initial bound state before dissociation, while scattering coordinates are applied for wave functions on the dissociative surface. These natural choices lead to three-dimensional nonseparable bound–continuum transition amplitudes. The translational continuum wave function is further approximated using Airy functions, and additional approximations are introduced based on the presence of small amplitude vibrations in the initial bound state. The three-dimensional transition amplitudes are then analytically reduced to sums of one-dimensional quadratures. The theory has been applied to the photodissociation of several rovibronic states of N2O+(Ã 2∑+) (predissociation) and ICN(C̃ 1A′) (direct photodissociation), and the rotational distributions for J=0 are in good agreement with three-dimensional close-coupled calculations except when the potentials become highly anisotropic. Our photodissociation infinite order sudden approximation is tested against various versions of the rotational infinite order sudden approximation for N2O+ and are found to be in good agreement with previous results. The present theory readily permits calculations for J>0 and may be used for the calculation of rotational distributions for excited rotational and/or vibrational states. In the limit of isotropic potentials the remaining integrals are evaluated to provide analytical approximations for the transition amplitudes.

The Mechanism of Occurrence of Dual Fluorescence in Intermediate Case Molecules as Viewed From Rotational Effects

Fluorescence decays of jet‐cooled pyrazine excited at various positions of rovibronic transitions within the 0–0 band were measured by the use of a pulsed dye laser pumped by an excimer or a nitrogen laser. The experimental data thus obtained on the number (N) of the triplet states coupled to the initially excited singlet rovibronic state were analyzed and interpreted by the aid of computer simulation using an asymmetric rotor program. The results indicate that the extraordinarily great variation of N with excitation energy and unreasonably small values of N hitherto reported in the literature can be understood in terms of the conventional theory based on the singlet–triplet mixed state model which has been developed for elucidating the mechanism of the occurrence of dual fluorescence in intermediate case molecules.

Methods for introducing vibrational effects in the calculation of electric dipole polarizabilities and hyperpolarizabilities (with reference to H+2)

Three different methods for taking into account nuclear motion while calculating molecular static dipole polarizabilities and hyperpolarizabilities are discussed. Within these methods both the purely variational and the variational-perturbational techniques are used to obtain numerical results for the α zz and γ zzzz polarizability components of the H+ 2 molecular ion in its rovibronic ground state.