Excited Vibronic States Research Articles

Dissociative recombination of electrons and molecular ions

The existing techniques for the calculation of the dissociative recombination (DR) of electrons and molecular ions were compared. The advantages of the method of multichannel quantum defect (MQD), in which equations are formulated directly for the T-matrix of collisions and the unitarity of the scattering S-matrix is thus ensured, were demonstrated. The effect of molecular rotation and of the nonadiabatic electron-rotation coupling on the e− + H2+ →, H* + H reaction was investigated. A procedure was suggested based on the use of the adiabatic approximation (with respect to the nuclear rotation) in the near-threshold area while taking into account the contributions of the excited vibronic states of the Rydberg complex formed in an intermediate stage of the reaction. It is notable that the partial rate constants (and the corresponding cross-sections) arc very sensitive to the initial rotation excitation. However, the temperature-averaged rate constants under equilibrium conditions are only slightly affected by rotation.

Rotationally resolved vibronic spectra of the van der Waals modes of benzene–Ar and benzene–Kr complexes

Rotationally resolved vibronic spectra of eight van der Waals bands built onto the 610 transition of the bare molecule are reported for the complexes C6H6⋅Ar, C6D6⋅Ar, and C6H6⋅84Kr. The rotational structure of most of the bands is identified as that of a perpendicular transition with Coriolis coupling constants nearly the same as those of the 610 band of the respective complex. We therefore conclude that the excited van der Waals modes of the three complexes have a1 symmetry. Precise rotational constants are fitted to the large number of unblended lines assigned in each spectrum. In contrast, the lowest energy van der Waals bands of both C6H6⋅Ar and C6D6⋅Ar display a completely different rotational structure which can neither be explained by a genuine perpendicular nor a genuine parallel transition. This situation will be analyzed in detail in accompanying work and the final vibronic assignments deduced. The rovibronic lines in all the spectra show a linewidth of 130 MHz that is solely due to the laser linewidth and to residual Doppler broadening in the molecular jet. It is concluded that the excited vibronic combination states of intramolecular and van der Waals vibrations do not predissociate on the nanosecond time scale of our experiment. Two of the reported spectra show irregularities in the rotational structure that are explained by coupling to adjacent combination states.

The Visible and Pure Rotational Spectrum of Platinum Monosulfide, PtS

Numerous band systems in the visible spectral region of a supersonic molecular beam sample were detected using medium-resolution laser-induced fluorescence spectroscopy. Three of the intense band systems were recorded at high resolution and rotationally analyzed. Pure rotational transitions in the ground state of 194PtS, 195PtS, and 196PtS were recorded using the pump/probe microwave optical double-resonance technique. Spectroscopic constants for the ground and three low-lying excited vibronic states have been derived.

Electric properties of organic molecules. The ground and excited vibronic state dipole moments of 1-fluoronaphthalene

The jet-cooled rotational spectrum of the A 1A′←X 1A′ 0 0 0 vibronic band of 1-fluoronaphthalene at 313.8 nm is recorded by scanning the frequency-doubled output of a cw ring dye laser. Stark shifts and splittings of selected rotational lines are measured by applying homogenous electric fields of the order of a few kV/cm. A change from the expected quadratic Stark effect to a linear behaviour is observed. Diagonalization of both rotational and Stark Hamiltonians is carried out with varying dipole moment components. The best fit is achieved for μ a =0.622(6) D, μ b =1.261(10) D, ¦μ¦=1.406(10) D in the X state and μ a =0.519(5) D, μ b =1.283(14) D, ¦μ¦=1.384(14) D in the A state.

Non-radiative fast processes and quantum efficiency of transition metal ions in soloids

In this contribution we investigate the dependence of quantum efficiency of Ti 3+ :Al 2O 3 on the wavelength of the exciting light. We consider fast non-radiative transitions between the highly excited vibronic states of the system and slow non-radiative and radiative processes which take place in the system thermalized in the first excited state. The results of our model calculations are compared with photopyroelectric spectra of Ti 3+ : sapphire.

Rotational Coherence Spectroscopy and Excited‐State Dynamics of Tolane and its Van Der Waals Complexes with Argon and Nitrogen

We report the results of rotational coherence spectroscopy on tolane, tolane–Ar, and tolane–N2. The results on tolane are consistent with a planar geometry for the species. They also provide information about the symmetries of excited vibronic states in the species. The results on the van der Waals complexes provide significant experimental constraints on where the small species bind to tolane. In addition, we report the observation of a rapid excited‐state decay process in the complexes and discuss the possible nature of this process.

Antisymmetric resonant vibrational Raman scattering

Antisymmetric resonant vibrational Raman scattering is studied using time-reversal symmetry. Under the Born-Oppenheimer approximation, for even-electron systems with the ground and the excited vibronic states belonging to real representations, the antisymmetric resonant Raman transition polarizabilities are strictly zero due to time-reversal symmetry. Only if the ground or the resonant excited vibronic states of an even-electron system belong to E representations of groups C n , C nh or S n ( n ⩾ 3), or for an odd-electron system, can the antisymmetric vibrational resonant Raman transition polarizability be nonzero. The results explain the inconsistency between the space symmetry prediction and the experimental facts of antisymmetric resonant Raman scattering.

Vibronic spectra and S1 excited electronic state molecular structures for acetyl chloride and acetyl fluoride

Vapor-state absorption spectra have been recorded for acetyl fluoride and acetyl chloride and also for deuterated derivatives with path lengths up to 40 m. The origins of the S1←S0 transitions have been derived, together with the torsional-vibration energy levels in the ground state S0 and excited singlet state S1. Fitting the calculated and observed rotational contours of the vibronic bands has been used to estimate the geometrical parameters in the S1 states. The carbonyl groups in the S1 states are nonplanar. The internal-rotation potentials have been determined for acetyl fluoride and acetyl chloride in the S1 and S0 states. The relative intensities of the torsional transitions in those states indicate that the minima in the potential energy are appreciably displaced along the torsional coordinate in the S0 and S1 states.

Vibronic energy map and excited state vibrational characteristics of magnesium myoglobin determined by energy-selective fluorescence.

The vibrational frequencies of the singlet excited state of Mg-substituted myoglobin and relative absorption probabilities were determined by fluorescence line-narrowing spectroscopy. These spectra contain information on the structure of the excited state species, and the availability of vibrationally resolved spectra from excited state biomolecules should aid in elucidating their structure and reactivity.

Integral cross sections for the reaction N+2+X2→N2X++X(X=H,D): A low energy crossed-beam experiment

Using a high-resolution crossed-beam apparatus, we have measured the integral cross section for the title reactions in the collision energy range from 0.025 to about 2 eV. Structure in the energy dependence of the cross section has been observed for both reactions. However, features for H2 and D2 appear at different collision energies. This has been attributed to the successive opening, as collision energy increases, of new adiabatic reactive channels via different vibronic states of the intermediate charge transfer complex N2+X2+(v) (X=H,D). Calculations of model potential energy surfaces show that the reactants’ surface does not cross the vibronic ground state. Therefore reactions proceed only via excited vibronic states resulting, in agreement with data from previous experiments, in vibrationally excited products.

Photoinitiated charge transfer in N2O+–Ar

Vibrationally structured electronic transitions of N2O+–Ar have been observed by measuring the wavelength-dependent yields of the photodissociation reactions to yield N2O+ or Ar+. There appear to be four structured overlapping electronic band systems which are distinguished by vibrational spacings and by their propensity towards production of either N2O+ or Ar+. Variations in the Ar+/N2O+ photoproduct ratio with wavelength are explained as due to vibrational predissociation on different potential-energy surfaces correlating with either Ar+ or N2O+ products. The first band system, observed exclusively at the N2O mass, has its origin close to 445 nm, corresponding approximately to the difference in the energies of N2O+[X 2Π3/2]+Ar[1S0] and N2O[1Σ+]+Ar+[2P3/2] and is assigned as an intracluster charge-transfer transition. Two strong band systems situated to higher energy are assigned as transitions to the two additional electronic states which are expected to correlate with 2P3/2 and 2P1/2 Ar+ and N2O[1Σ+] products. While excitation of these two bands results almost exclusively in Ar+ production, a fourth weaker band near 342 nm leads to N2O+ and appears likely to be a transition to a state correlating with an excited vibronic state of N2O+[A 2Σ+(1,0,0)]+Ar[1S0]. The different band systems exhibit extensive vibrational progressions involving the deformation of the bond between the N2O and the Ar. The shift in the onset of the first charge transfer from the difference in the Ar and N2O ionization potentials combined with the appearance energy for Ar+ production allow tentative estimates of 690 and 1340 cm−1 to be made for the dissociation energies of the lowest and first excited states of N2O+–Ar.

Human serum albumin-benzo[a]pyrene anti-diol epoxide adduct structure elucidation by fluorescence line narrowing spectroscopy.

Cryogenic (4-10 K) laser-induced vibrationless ground state and vibronic excited state fluorescence emission spectra of the adducts resulting from reaction in vitro of human serum albumin and the carcinogen (+-)-r-7,t-8-dihydroxy-c-9,c-10-epoxy-7,8,9,10- tetrahydrobenzo[a]-pyrene were recorded in order to determine the structures formed. Comparison of these fluorescence line-narrowed (FLN) spectra to those obtained from BaP-7,8,9,10- tetrahydrotetrols, synthetic N-t-BOC-alaninate ester, and N tau- and N pi-histidine amine anti-BaPDE adducts revealed that a mixture of adduct types are formed with the protein. Extensive dialysis of the adducted protein simplified the FLN spectrum, causing it to become nearly identical to the FLN spectrum obtained from the stable peptide adduct. Comparison of the FLN spectra of the synthetic histidine adducts to those obtained from peptide adducts isolated from enzymic digestion of the adducted protein indicated that only one of the imidazole nitrogens is the nucleophile which forms a stable adduct with anti-BaPDE. The FLN studies confirm that N tau-histidine adducts are formed between human serum albumin and the C-10 position of anti-BaPDE.

Millimeter-wave spectrum of NCS radical in the ground 2Π state

The rotational transitions of NCS have been observed in the 1 to 3 mm wavelength region in a hollow cathode discharge in a gas mixture of CS2 (∼1 mTorr) and N2 (∼30 mTorr). The molecular constants in the ground state are determined by fitting the observed frequencies to the standard Hamiltonian for 2Π states. The accuracy of the molecular constants are greatly improved compared with the values obtained from the optical data. The lines in the excited vibronic states have also been observed. The analysis including those excited state lines is in progress and the results will be published separately.

Van der Waals bond lengths and electronic spectral shifts of the benzeneKr and benzeneXe complexes

Rotationally resolved UV-spectra are presented for the 6 1 0 bands of benzeneKr and benzeneXe complexes yielding precise rotational constants and van der Waals bond lengths for the ground and excited vibronic state, and electronic band shifts. These value complement the previously published data for the other rare gases and the various quantities have now been determined for all the benzene—rare gas complexes. Measured values of the bond length were used to calculate the band shifts from recent theoretical predictions. They are compared with the experimental values of this work.

Study of dark states in naphthalene, pyrimidine and pyrazine by detection of phosphorescence after UV laser excitation

The high-resolution fluorescence and phosphorescence detected spectra after excitation of the (3a g) 1 and (4a g) 1 vibronic states of the S 1 ( 1B 3u) in naphthalene have been studied in a molecular beam. The residual Doppler linewidth of 20 MHz allowed rotational resolution. All lines were assigned and the rotational constants of the (3a g) 1 and (4a g) 1 states were determined. It has been shown that the intersystem crossing rates are independent of the rotational quantum numbers J and K up to J=10 in both excited vibronic states. The strong ISC rate of the (4a g) 1 state is confirmed. No evidence was found for a promoting second triplet T 2 state. Detection of phosphorescence after excitation of the vibrationless S 1 ( 1B 3u) state in pyrazine and the S 1 ( 1B 1) s pyrimidine yielded no signal. This observation confirms the assumption that after excitation of the S 1 state the energy is transferred to high vibrational states of the S 0 state.

Raman excitation profiles and second-derivative absorption spectra of β-carotene

The absorption and second-derivative spectra of β-carotene in n.hexane, isopentane, and CS2, as well as the Raman excitation profiles for six modes in n.hexane and isopentane have been measured. The combined analysis of the absorption and second-derivative spectra in terms of model summing over the vibronic excited states furnished reliable values of the excited state displacement parameters. The input parameters for the calculation of the full spectra were taken from the Raman band relative intensities, while the line shape functions and their widths were determined by the second-derivative absorption spectra. The Raman excitation profiles were then calculated in terms of the transform theory, both the observed and calculated absorption spectra being transformed. Very good fits with the experimental profiles were found by including a small contribution of inhomogeneous broadening to the total width. It was concluded that the homogeneous broadening mainly derives from dephasing of nondegenerate low-frequency vibrations in the resonant electronic excited state. The low temperature profiles were easily calculated by decreasing this homogeneous contribution. The agreement between calculated and experimental results was improved by assuming that the vibrational frequencies of the carbon–carbon stretching modes increase at the excited state. The resulting nuclear diplacements and frequency shifts at the excited state as well as the Gaussian widths correlate very well with the absorption spectral properties of linear polyenes.

Observation of the formation of C+, O+, and ArC+ in the collisions of Ar+(2P3/2,1/2) with CO

Absolute total cross sections for the formation of C+ , O+ , and ArC+ in the collisions of Ar+ (2 P3/2,1/2 )+CO have been measured over the center-of-mass collision energy (Ec.m. ) range of ∼4–123 eV. The observed appearance energies for C+ and O+ are in agreement with the thermochemical thresholds for the C+ (2 P)+O(3 P) and O+ (4 S0 )+C(3 P) product channels, respectively. The cross sections for C+ are significant compared to those for CO+ . At Ec.m. =12.9 eV, the analysis of the kinetic energy distributions of C+ and CO+ supports the conclusions that many excited vibronic states of CO+ are populated and that the C+ ions are formed by predissociation of electronic excited CO+ .

Excited state dipole moments and polarizabilities of centrosymmetric and dimeric molecules. III. Model calculations for 1,8-diphenyl-1,3,5,7-octatetraene

A vibronic coupling model is used for the quantitative interpretation of both the 1 1B u ← 1 1A g absorption and the 2 1A g → 1 1A g fluorescence spectra of 1,8-diphenyl-1,3,5,7-octatetraene (DPO) in cyclohexane solution at 298 K in the absence and in the presence of an external electric field (electro-optical spectra). It is assumed that the coupling of the close lying 2 1A g and 1 1B u states is mainly due to static perturbations by the solvent environment and occurs via two totally symmetric CC and CC bond stretching vibrations. The solvent perturbations give rise to non-zero dipole moments of the vibronic excited states and influence the excited state polarizabilities. Both quantities strongly depend on the considered vibronic level. This implies that the electro-optical absorption band is not homogeneous with respect to the electro-optical properties of the underlying transitions. Some general aspects of the evaluation of heterogeneous electro-optical spectra are discussed for a two-band system. The long axis polarizability of the fluorescent state of DPO is determined by integral electro-optical emission measurements.

The temperature dependence of the EPR spectrum of Cu2+ in ZnTiF6⋅6H2O between 4 and 160 K

The EPR spectrum of Cu2+ in the low temperature monoclinic phase of ZnTiF6⋅6H2O was studied at 9.24 GHz between 4 and 160 K. Assuming the Jahn–Teller spectra to have axial symmetry, g∥ was found to vary from 2.472±0.004 at 4 K to 2.331±0.004 at 158 K, while g⊥ varied from 2.097±0.004 to 2.173±0.004 over this range. From the temperature variations of the g values, the lowering of the energy Δ of one potential well associated with the Jahn–Teller distortion compared to the other two was found to be approximately 143 cm−1. It is shown that an apparent decrease in Δ with increasing temperature above 100 K could be due to the presence of an excited vibronic state about 170 cm−1 above the ground state. The variation of ‖A∥‖ from (107±1)×10−4 cm−1 at 4 K to (53±3)×10−4 cm−1 at 158 K agreed with predictions based on the temperature variation of the g tensor.

Ab initio MRD-CI Calculations for the decay of molecular excited states via radiative and dissociative mechanisms

The ab initio multi-reference single- and double-excitation configuration interaction (MRD-CI) method for the calculation of the energies and properties of electronically excited states is employed to investigate a variety of molecular decay processes. The theoretical basis for the prediction of oscillator strengths and radiative lifetimes is reviewed and the computational problems peculiar to the calculation of matrix elements between different electronic states are discussed. A number of applications is considered for the prediction of radiative lifetimes of molecular excited states and comparison with experimental data is made for C 2, HSO, SiH, C 3 and other systems in which dipole-allowed transitions are involved. Changes in the computational procedure which are required for the treatment of spin-forbidden and otherwise dipole-forbidden transitions are described and some examples for O 2 and related systems are considered. Decay processes are then considered which involve predominantly nuclear motion and the manner in which such phenomena are described quantum mechanically is compared with the methodology employed for radiative decay. Various examples of photodissociation reactions are discussed with emphasis on the different types of mechanisms which commonly occur in such processes: explicit calculations for the systems methanol, HO 2 and O 2 + are considered in this case. The role of non-adiabatic effects in such dissociative processes is investigated in detail and additional cases in which ab initio treatments have been carried out to demonstrate the importance of this type of mechanism are reviewed. Finally a procedure is described for revising the existing MRD-CI program system to allow for the direct computation of natural lifetimes (line widths) of vibronic excited states in terms of complex eigenvalues of non-Hermitian matrices, as prescribed by the Complex Coordinate Method and/or the Direct Siegert Method.