Vibrational excitation of deuterium fluoride and hydrogen fluoride by atomic and diatomic species

The vibrational excitation of HF and DF and the energy transfer efficiencies for various collision partners were investigated over the temperature and pressure ranges of 1400°K to 4100°K and 0.1 to 0.3 atm, respectively. The extent of excitation was determined as a function of time by continuously monitoring the infrared emission intensity at the center of the 1–0 vibration-rotation band of the molecule. Collisional efficiencies of HF, N₂, O₂, F, Cl, and DF in relaxing HF and of DF, HF, and N₂ in relaxing DF are reported. A comparison with relaxation data for pure HF taken at lower temperature suggests that long-range attractive forces are mechanistically of major importance in the relaxation process. The relatively high efficiency of atomic chlorine in relaxing HF, i.e., (τP)_HF–HF/(τP)_HF–C1 ≥ 5 at 3000°K is discussed in terms of our previous result for atomic fluorine, i.e., (τP)_HF–HF/(τP)_HF–F = 18.     

Deactivation of vibrationally excited hydrogen fluoride (v=2 and v=1) by atomic fluorine

Measurements have been made which provide information concerning the rate of energy transfer between vibrationally excited HF and atomic fluorine. It was found that HF (v = 1) deactivates about seven times slower than was determined in previous experiments. The deactivation rate constant for HF (v = 2) is about six times larger than for HF (v = 1).     

Unidirectional electronic ring current driven by a few cycle circularly polarized laser pulse: quantum model simulations for Mg-porphyrin

A circularly polarized ultraviolet (UV) laser pulse may excite a unidirectional valence-type electronic ring current in an oriented molecule, within the pulse duration of a few femtoseconds (e.g., tau = 3.5 fs). The mechanism is demonstrated by quantum model simulation for |X = |1(1)A(1g) --> |E(+) = |4 (1)E(u+) population transfer in the model system, Mg-porphyrin. The net ring current generated by the laser pulse (I = 84.5 microA) is at least 100 times stronger than any ring current, which could be induced by means of permanent magnetic fields, with present technology.     

Steering Vibrational Population Transfer via Double-Σ-Type Laser Scheme

The vibrational state-selected population transfer from a highly vibrationally excited level to the ground level is of great importance in the preparation of ultra-cold molecules. By using the time-dependent quantum-wave-packet method, the population transfer dynamics is investigated theoretically for the HF molecule. A double-Σ-type laser scheme is proposed to transfer the population from the |v=16> level to the ground vibrational level |v=0> on the ground electronic state. The scheme consists of two steps: The first step is to transfer the population from |v=16> to |v=7> via an intermediate level |v=11>, and the second one is to transfer the population from |v=7> to |v=0> via |v=3>. In each step, three vibrational levels form a Σ-type population transfer path under the action of two temporally overlapped laser pulses. The maximal population-transfer efficiency is obtained by optimizing the laser inten-sities, frequencies, and relative delays. Cases for the pulses in intuitive and counterintuitive sequences are both calculated and compared. It is found that for both cases the population can be efficiently (over 90%) transferred from the |v=16> level to the |v=0> level.     

Perturbation for a rigid rotator in an electric field

The behavior of a rigid rotor perturbed by an electric field is studied. An hypervirial calculation, to obtain the perturbation series without calculation of the perturbed wavefunctions, is used to determine the Stark-shifted eigenvalues. A numerical estimation of the m equals; 0 → m = 1 transition for the lowest vibrational state (v = 0) and the first rotational state (J = 1) for the HF and DF molecules using Padé approximants is made.     

Energy resonance and inverse population of the product vibrational states for the reaction F + H2(v = 0) → HF(v′) + H,LCAC‐SW theoretical quantum scattering study

LCAC‐SW (linear combination of arrangement channel‐scattering wavefunction) method was used to calculate collinear state‐to‐state reaction probabilities for the reaction F + H₂(v = 0) → HF(v′) + H on the 6SEC potential energy surface. The results show that reaction probabilities P₀₂ and P₀₃ [i. e., v′ = 2,3 for reaction F + H₂ (v = 0) + HF(v′) + H] are primary, the population of product vibrational states is inverse and the reaction probabilities are oscillatory with collision energies, i.e., there is energy resonance in this reaction, which agrees with a new experiment.     

Femtosecond dynamics of electronic excitations of adsorbates studied by two-photon photoemission pulse correlation: CO/Cu(111)

An equal pulse correlation technique based on angle-resolved two-photon photoemission is employed to investigate the lifetime of electronic excitations of an adsorbate on a single crystal metal surface. Photoemission from an occupied surface state on Cu(111) by a non-resonant two-photon process via the sp-band gap is used to characterize directly at the surface the width and shape of 65 fs laser pulses at a photon energy of hv = 3.4 eV. The 2PPE correlation technique allows the establishment of an upper limit of τ < 20 fs for the lifetime of the 2π^* resonance of CO adsorbed on Cu(111), whereas from the spectral width of the 2π^* level a lower limit of ≈1 fs is estimated.     

Experimental and Theoretical Investigation of the 3sp(d) Rydberg States of Fenchone by Polarized Laser Resonance-Enhanced-Multiphoton-Ionization and Fourier Transform VUV Absorption Spectroscopy

The VUV absorption spectrum of fenchone is re-examined using synchrotron radiation Fourier transform spectrometry, revealing new vibrational structure. Picosecond laser (2+1) resonance enhanced multiphoton ionization (REMPI) spectroscopy complements this, providing an alternative view of the 3spd Rydberg excitation region. These spectra display broadly similar appearance, with minor differences that are largely explained by referring to calculated one- and two-photon electronic excitation cross-sections. Both show good agreement with Franck-Condon simulations of the relevant vibrational structures. Parent ion REMPI ionization yields with both femtosecond and picosecond excitation laser pulses are studied as a function of laser polarization and intensity, the latter providing insight into the relative two-photon excitation and one-photon ionization rates. The experimental circular-linear dichroism observed in the parent ion yields varies strongly between the 3s and 3p Rydberg states, in good overall agreement with the calculated two-photon excitation circular-linear dichroism, while corroborating other evidence that the 3p_z sub-state plays no more than a very minor role in the (2+1) REMPI spectrum. Vibrationally resolved photoelectron spectra are recorded with picosecond pulse duration (2+1) REMPI at selected intermediate vibrational excitations. The 3s intermediate state displays a very strong Δv=0 propensity on ionization, but the 3p intermediate evidences more complex vibronic dynamics, and we infer some 3p→3s internal conversion prior to ionization.     

Relaxation of DF(v = 1) by various polyatomic molecules

By means of the technique of laser-induced fluorescence, the room-temperature vibrational relaxation of DF(v = 1) has been studied in the presence of several polyatomic chaperones. The rate coefficients obtained [in units of (μ;sec·torr)^?1] are CH₄, 0.22; C₂H₆, 0.61; C₄H₁₀, 1.26; C₂H₂, 4.0 × 10^?2; C₂H₂F₂, 1.86 × 10^?2; C₂H₄, 0.175; CH₃F, 0.36; CF₃H, 1.95 × 10^?2; CF₄, 1.0 × 10^?3; CBrF₃, 5.6 × 10^?4; NF₃, 5.1 × 10^?4; SO₂, 1.27 × 10^?2; and BF₃, 7.1 × 10^?3. Results are also reported for vibrational relaxation rate coefficients for HF(v = 1) in the presence of the following chaperones: CH₄, 2.6 × 10^?2; C₂H₆, 5.9 × 10^?2; C₃H₈, 8.4 × 10^?2; and C₄H₁₀, 0.128. A comparison of DF and HF results indicates that for deactivation by C_nH_n+2, rate coefficients for DF are approximately an order of magnitude larger than for HF. The deactivation rate coefficient of DF(v = 1) by CH₄ was found to decrease with increasing temperature between 300 and 740°K.     

Determination of flurazepam and its hydroxyethyl and dealkyl metabolites by some packed and capillary ECD-GLC methods: Application to pharmacokinetic studies

A sensitive gas-liquid chromatographic assay was developed for plasma determinations of flurazepam (F), N1-desalkylflurazepam (DF), and N1-hydroxyethylflurazepam (HF). The internal standards used were delorazepam for F and DF and 1 ã-hydroxymidazolam for HF. Following extraction and derivatization for HF, these compounds were analyzed by ECD-GLC, using either glass packed columns (poly A 103 for F and DF, OV 225 for HF) or CP-Sil 5 capillary columns for DF and HF. The detection limit of the packed-column assay was approximately 3ng/ml for HF and DF, and 5ng/ml for F; with capillary GLC analysis, the sensitivity limit for HF and DF was lower than 1 ng/ml. These methods were used for pharmacokinetic investigations after i. v. administration of these compunds.     

Vibrational deactivation of DF

Rate constants for deactivation of DF in vibrational states n = 1 to 7 in collisions with DF (0) are calculated semiclassically in the temperature range from 300 K to 3000 K. The variation with the temperature agrees quite well with experiments for n = 1, although the theoretical rates are 30% lower than the experimental values for temperatures above 1200 K and 75% lower at 300 K. This difference is discussed. Single quantum transitions dominate. At 300 K the mechanism is predominantly a V—V transfer for n = 2 and a V-T/R transfer for n = 7, while both mechanisms contribute for n = 3–6. Collision complexes are important for both V—V and V-T/R energy transfer. Rotational relaxation times are calculated for HF and DF.     

Ultrafast photodissociation dynamics of HMX and RDX from their excited electronic states via femtosecond laser pump–probe techniques

Femtosecond laser pump–probe techniques are employed to investigate the mechanisms and dynamics of the photodissociation of HMX and RDX from their excited electronic states at three wavelengths (230 nm, 228 nm, and 226 nm). The only observed product is the NO molecule. Parent HMX and RDX ions are not observed. The NO molecule has a resonant A²Σ ← X²Π (0, 0) transition at 226 nm and off-resonance two-photon absorption at 228 nm and 230 nm. Pump–probe transients of the NO product at both off-resonance and resonance absorption wavelengths indicate the decomposition dynamics of HMX and RDX falls into the timescale of our laser pulse duration (180 fs).     

Ab initio HF-SCF study of naphthazarin: Geometries,isomerism, hydrogen bonding,and vibrational spectrum

The structural properties and intramolecular hydrogen bonding of a series of structures of naphthazarin molecule were investigated by ab initio HF-SCF methods. The geometries of theC _2v ,C _2h ,D _2h , andC _s symmetry structures were optimized using split-valence basis sets. MP2/6-31G^*// HF/6-31G single-point energy calculations indicate that theC _2v isomer (5,8-dihydroxy-1,4-naphthoquinone) is the lowest energy structure of the molecule and that theC _2h symmetry one (4,8-dihydroxy-1,5-naphthoquinone), lying 37 kJ/mol above theC _2v form, is the other stable isomer of naphthazarin. At the HF/6-31G level, the intramolecular proton exchange between two equivalentC _2v structures is a two-step process where each proton can be independently transferred through an unsymmetrical potential having a 1,5-quinone intermediate, theC _2h symmetry structure, and two equivalent transition states ofC _s symmetry, with a barrier height equal to 38 kJ/ mol (MP2/6-31G^*//HF/6-31G). The study of naphthazarin molecule is flanked by a theoretical investigation on theC _2v andC _2h isomers of the parent naphthoquinone and dihydroxynaphthalene molecules. The SCF vibrational spectrum of the ground state of naphthazarin, harmonic frequencies, and infrared and Raman band intensities were computed at the HF/6-31G level. The results of the calculations are compared with the matrix isolation FT-IR spectroscopy measurements and with the infrared and Raman spectra of the crystal molecule.     

Vibrational and rotational energy transfer in Ar + HF

State-to-state energy transfer cross sections for Ar + HF (v = 2, 4, and 6; J = 4, 6, 8, and 10) were computed using quasiclassical trajectories. Rotational energy transfer is invariant with increasing v, but vibrational energy transfer is significantly enhanced by increasing J.     

Quantum Dynamics of Li+HF/DF Reaction Investigated by a State-to-State Time-dependent Wave Packet Approach

Using the reactant coordinate based time-dependent wave packet method, on the APW potential energy surface, the differential and integral cross sections of the Li+DF/HF(v=0, j=0, 1) reactions were calculated over the collision energy range from the threshold to 0.25 eV. The initial state-specified reaction rate constants of the title reaction were also calculated. The results indicate that, compared with the Li+DF reaction, the product LiF of Li+HF reaction is a little more rotationally excited but essentially similar. The initial rotational excitation from j=0 to 1 has little effect on the Li+DF reaction. However, the rotational excitation of DF does result in a little more rotationally excited product LiF. The different cross section of both reactions is forward biased in the studied collision energy range, especially at relatively high collision energy. The resonances in the Li+HF reaction may be identifiable as the oscillations in the product ro-vibrational state-resolved integral cross sections and backward scattering as a function of collusion energy. For the Li+HF reaction, the rate constant is not sensitive to the temperature and almost has no change in the temperature range considered. For the Li+DF reaction, the rate constant increase by a factor of about 10 in the temperature range of 100?300 K. Brief comparison for the total reaction probabilities and integral cross section of the Li+HF reaction has been carried out between ours and the values reported previously. The agreement is good, and the difference should come from the better convergence of our present calculations.     

Multiphoton-Excited Visible Emission by Serotonin Solutions

Nonlinear excitation of the neurotransmitter serotonin (5HT) in aqueous solution is shown to generate a blue-green-emitting photoproduct in addition to UV fluorescence characteristic of native 5HT. The visible emission rate in diffusional steady-state measurements scales as the sixth power of excitation intensity, demonstrating that absorption of six near-IR photons is required to generate emission of one visible photon. Transient measurements reveal that this process is composed of two sequential nonlinear steps, the first excited by four photons and the second by two photons. These results, in combination with measurements of multiphoton-excited serotonin UV fluorescence, support a model in which 5HT is photochemically transformed as a consequence of four-photon absorption (E_tot?6 eV) to a photoproduct that then emits in the visible region via two-photon excitation. A minimum bound of ?10^-51 cm⁴ s photon^-1 is observed for the two-photon emission action cross section at 830 nm. Photoionization, rather than reaction with a dissolved oxygen species, appears to be the primary mechanism for generation of the blue-green-emitting photoproduct. The peak intensities required to generate significant blue-green emission (?5 times 10¹¹ W cm^-2 from 80 MHz 150 fs titanium: sapphire laser pulses) are approximately five-fold higher than are typically used in two-photon laser scanning microscopy but are still substantially lower than the estimated intensity needed to induce dielectric breakdown of water.     

Vibrational energy disposal in reactions of fluorine atoms with hydrides of groups III,IV and V

The HF infrared chemiluminescence from the reactions of F atoms with B₂H₆, CH₄, CH₃F, CH₂F₂, CH₂Cl₂, CH₃ONO. CH₃NO₂, NH₃ (and ND₃). PH₃ and HNCO has been observed from a 300 K flowing-afterglow reactor. Experiments were done for a range of CH₄ and F atom concentrations to identify conditions which were free of vibrational relaxation and secondary reactions, and these conditions were used to assign initial HF(v) vibrational distributions for each reaction. The emission intensity from each reaction also was compared to that from CH₄ in order to obtain the relative HF formation rate constants at 300 K. Since the absolute rate constant for F + CH₄ is well established, the combination of all of these data provides absolute rate constants for HF(v) formation at 300 K. The ND₃ reaction was studied to obtain information on more vibrational levels in order to better estimate the HF(v = 0) and DF(v = 0) components of the ammonia distributions. With NH₃ and ND₃ there is no significant isotope effect on the energy disposal. Except for NHCO, for which an addition-elimination channel is possible, the HF(v) distributions are inverted and <f_v > = 0.60. Differences between the HF(v) distributions reported here and some other reports in the literature are noted: the present data are discussed as representative of direct H atom abstraction for 300 K Boltzmann conditions. The HCl infrared chemiluminescence from the F + CHCl₂ secondary reaction also was observed; the HCl(v) distribution was v₁: v₂: v₃: v₄: v₅ - 0.47: 0.23: 0.18: 0.08: 0.04.     

Femtosecond vibrational nuclear dynamics in the electronically excited state of the I2 molecule

Dynamics of the nuclear motion in the bound electronic B-state of the I₂ molecule was studied in the real time scale. Experiments were performed by the femtosecond pump-probe technique, which measured the dependence of the intensity of fluorescenceP(t) in the highly excited f-state on the time delay between pump and probe pulses. TheP(t) dependence observed has an oscillating character with a period of –300 fs. The pump pulse was generated by a femtosecond dye laser and amplified in a pulse dye laser amplifier; its spectral width was 5.6 nm, the wavelength in the center of the spectrum was 614 nm, and the duration was 90 fs. The probe pulse was generated in a KDP crystal due to duplication of the light frequency; its spectral width was 1.2 nm, the wavelength in the center of the spectrum was 307 nm, and the duration was 120 fs. TheP(t) dependence on the parameters of the probe and pump pulses was theoretically analyzed in terms of the quantum model based on the known energies of electronic vibrational-rotational states in the X-, B-, and f-terms of the iodine molecule. Experimental and calculatedP(t) plots at time delays of up to l.5 ps and time resolution of less than 100 fs were compared. Values of potentials in the X-, B-, and f-terms of the iodine molecule, spectra, and durations of pump and probe pulses are sufficient data for quantitative calculation of the experimentally obtainedP(t) plot.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 594–600, March, 1996.     

Molecular frame photoelectron angular distributions for H2 ionization by single and trains of attosecond XUV laser pulses

The delayed autoionization of H₂ doubly excited states into channels of different inversion symmetry gerade and ungerade is investigated by using pulses of attosecond duration (isolated or packed in trains), linearly polarized along the molecular axis. It has been shown in previous work, by using XUV laser pulses with durations of 4 fs or longer, that the molecular frame photoelectron angular distributions (MFPAD) associated with the dissociative channel H⁺ + H(n?) are not symmetric with respect to the inversion center of the molecule. In contrast, the MFPADs become symmetric for shorter fs pulses. Here we show that, although this is still the case for pulses of attosecond duration, the combination of two of these pulses with a controlled time delay may still lead to asymmetric MFPADs. From the analysis of the time evolution of the calculated MFPADs, we propose a way to elucidate autoionization lifetimes of molecular resonant states. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2462–2471, 2010     

Photo-Induced Ultrafast Electron Dynamics in Anatase and Rutile TiO2: Effects of Electron-Phonon Interaction

The photo-induced ultrafast electron dynamics in both anatase and rutile TiO\begin{document}$_{2}$\end{document} are investigated by using the Boltzmann transport equation with the explicit incorporation of electron-phonon scattering rates. All structural parameters required for dynamic simulations are obtained from ab initio calculations. The results show that although the longitudinal optical modes significantly affect the electron energy relaxation dynamics in both phases due to strong Fr?hlich-type couplings, the detailed relaxation mechanisms have obvious differences. In the case of a single band, the energy relaxation time in anatase is 24.0 fs, twice longer than 11.8 fs in rutile. This discrepancy is explained by the different diffusion distributions over the electronic Bloch states and different scattering contributions from acoustic modes in the two phases. As for the multiple-band situation involving the lowest six conduction bands, the predicted overall relaxation times are about 47 fs and 57 fs in anatase and rutile, respectively, very different from the case of the single band. The slower relaxation in rutile is attributed to the existence of multiple rate-controlled steps during the dynamic process. The present findings may be helpful to control the electron dynamics for designing efficient TiO\begin{document}$_{2}$\end{document}-based devices.