Photoacoustic measurements of photofragmentation in CH3I

Photoacoustic measurements are described giving branching ratios for the I(²P_{$\text{1}\text{2}$}) and I(²P_{$\text{3}\text{2}$}) atom production following vapour-phase photolysis of CH₃I. A range of excitation wavelengths are used from the long wavelength tail up to 248 nm. The presence of three bands is shown within the σ^* ← n continuum; in the strong-coupling model these are E ← A₁(⊥), A^*₁ ← A₁(||) and E ← A₁(⊥) with only the A^*₁ ← A₁ transition giving excited iodine atoms.     

Communication: Barium ions and helium nanodroplets: Solvation and desolvation

The solvation of Ba(+) ions created by the photoionization of barium atoms located on the surface of helium nanodroplets has been investigated. The excitation spectra corresponding to the 6p (2)P(1∕2) ← 6s (2)S(1∕2) and 6p (2)P(3∕2) ← 6s (2)S(1∕2) transitions of Ba(+) are found to be identical to those recorded in bulk He II [H. J. Reyher, H. Bauer, C. Huber, R. Mayer, A. Schafer, and A. Winnacker, Phys. Lett. A 115, 238 (1986)], indicating that the ions formed at the surface of the helium droplets become fully solvated by the helium. Time-of-flight mass spectra suggest that following the excitation of the solvated Ba(+) ions, these are being ejected from the helium droplets either as bare Ba(+) ions or as small Ba(+)He(n) (n < 20) complexes.     

Laser photoassociation of Xe and Br(2P3/2) atoms and generation of aligned XeBr(B) molecules

The laser-induced photoassociation of colliding pairs of Xe and Br(²P_3/2) atoms has been demonstrated by observing the XeBr(B→A) fluorescence following the XeBr(B→X) laser-induced excitation. Analysis of the B←X excitation spectrum shows that the excitation transition is almost entirely bound — free in nature. The fluore scence and excitation XeBr* spectra are used to discuss the XeBr (X, B and A) potentials. Analysis of the polarization of the XeBr(B-X) fluorescence shows that the XeBr(B) molecules are generated with a high degree of alignment relative to the plane-polarized laser beam. The pressure dependence of the decay rate of the total intensity and of the polarization give radiation lifetimes, quenching rate constants and an estimate for the de-alignment cross section in collisions with Xe.     

Potentials of the D¹0u⁺ (6¹S₀) and F³1u(6³P₂) electronic Rydberg states of Cd₂ from ab initio calculations and laser-induced fluorescence excitation spectra

The method of supersonic free-jet expansion beam combined with techniques of laser spectroscopy was used in an investigation of vibronic and isotopic structures in the D10(u)? (61S?) and F31(u)(63P?) electronic energy Rydberg states of Cd?. Laser-induced fluorescence excitation spectra recorded using the D10(u)? ← X10(g)?(51S?) and F31(u) ← X10(g)? transitions in the region of 206-218 nm provided spectroscopic characteristics of the excited states and allowed constructing of their intratomic potentials. Isotopic structures recorded in the (υ',υ') bands of the D10(u)? ← X10(g)? transition were used in determination of the D10(u)? state vibrational characteristics (ω'(e)x'(e), ω'(e)x'(e)) and υ' assignment. The ν(0,0) recorded directly in the F31(u) ← X10(g)? transition enabled determination of the bottom of the F31(u) state potential well. Valence ab initio calculations of Cd? interatomic potentials were performed with relativistic and spin-orbit effects taken into account. The experimental results were compared with results of the ab initio calculations. A free-jet expansion of Cd? as a source of entangled atoms for a test of Bell's inequality was analyzed.     

Hahn- and stimulated photon-echo decay measurements in the lowest doublet-doublet electronic transition of triphenylmethyl in triphenylamine

The results of Hahn (two-pulse) and stimulated (three-pulse) photon-echo decay measurements in the lowest ²A ← ²A electronic transition of triphenylmethyl in triphenylamine are reported and discussed. From the Zeeman effect on the photon-echo relaxation behaviour we conclude that hyperfine and superhyperfine interactions play a dominant role in the optical (phase) relaxation process.     

聚炔烃电子吸收光谱的理论研究

应用DFT/B3LYP方法,在6-31G水平上计算了HC2nH(n=1～13)的基态平衡几何构型和振动频率.在基态平衡构型下,通过TD-B3LYP/cc-PVTZ计算,确定了HC2nH(n=1～5)体系电子跃迁的能量和对应的振子强度.根据计算结果并结合先前的价键研究导出了HC2nH聚炔烃体系电子跃迁能与体系大小n有关的解析表达式.     

Stimulated radiative dissociation and gain measurements of Xe2Cl in solid xenon

The molecular charge transfer states of Cl-doped solid xenon form an ideal four-level laser system. UV excitation on the molecular XeCl (Bi ← X) pair potentials leads to the formation of the diatomic exciplex which relaxes with a nearly unity quantum yield to the triatomic Xe⁺₂ Cl⁻ (4²Γ) state. The radiative dissociation of the triatomic exciplex can be stimulated to provide monoenergetic atoms ≈ 1 eV above ground. While these systems are characterized by large gain coefficients, scattering losses predominate in samples prepared by standard matrix isolation techniques. Scattering losses are due to the inability of the lattice to accommodate the excess kinetic energy released in the bound to repulsive transition of the triatomic exciplex.     

Time-resolved intraband electronic relaxation dynamics of Hg(n) (-) clusters (n=7-13,15,18) excited at 1.0 eV

Time-resolved photoelectron imaging has been used to study the relaxation dynamics of small Hg(n) (-) clusters (n=7-13,15,18) following intraband electronic excitation at 1250 nm (1.0 eV). This study furthers our previous investigation of single electron, intraband relaxation dynamics in Hg(n) (-) clusters at 790 nm by exploring the dynamics of smaller clusters (n=7-10), as well as those of larger clusters (n=11-13,15,18) at a lower excitation energy. We measure relaxation time scales of 2-9 ps, two to three times faster than seen previously after 790 nm excitation of Hg(n) (-), n=11-18. These results, along with size-dependent trends in the absorption cross-section and photoelectron angular distribution anisotropy, suggest significant evolution of the cluster anion electronic structure in the size range studied here. Furthermore, the smallest clusters studied here exhibit 35-45 cm(-1) oscillations in pump-probe signal at earliest temporal delays that are interpreted as early coherent nuclear motion on the excited potential energy surfaces of these clusters. Evidence for evaporation of one or two Hg atoms is seen on a time scale of tens of picoseconds.     

Polarized fluorescence spectra of retinol ad diphenyl octatetraene

The polarization of fluorescence excitation and emission bands of all-trans retinol in 77°K has been measured at relatively high resolution. The main excitation band as well as the fluorescence band showed a degree of polarization approaching that the theoretical maximum of 0.5 for a linear oscillator under photoselective conditions. These results are consistent with the assignment that the main absorption and fluorescence bands originate from the long-axis polarized ⁱB ← ^IA transition, as expected from free electron theory. Diphenyl octatetraene was also studied, yielding results similar to the retinol system.     

Fluorescence spectra of matrix isolated silver atoms

Silver atoms isolated in noble gas matrices show fluorescence spectra with large Stokes shifts, when optically excited via the 5P ← 5S transition which is three-fold split in the matrix by spin—orbit and crystal field interaction. The shifts are explained by the formation of an excimeric bond between the excited metal atom and the noble gas cage. For Ag in Kr it is demonstrated, that most of the Stokes shift arises from the ground state repulsion.     

The influence of hyperfine structure and isotope shift on the detection of Rb by 2f-wavelength modulation diode laser absorption spectrometry-experimental verification of simulations

This work presents an experimental verification of a previously developed methodology for simulation of the 2f-wavelength modulation diode laser absorption spectrometry technique (2f-WM-DLAS) when the influence of hyperfine structure, isotope shift and collisional broadening and shift of an atomic transition is taken into account [J. Gustafsson, D. Rojas and O. Axner, Spectrochim. Acta, 52B, 1937–1953 (1997)]. The pilot element in the simulations was Rb, detected at the 780 nm 5s ²S_1/2−5p ²P_3/2 transition, in low-pressure cells and atmospheric-pressure reservoirs (e.g. graphite furnaces). This experimental investigation verifies that the simulations are able to predict, with good accuracy, experimental 2f-WM signals from Rb atoms under both low-pressure, room-temperature conditions and atmospheric-pressure, high-temperature conditions. This implies that the previously published simulation methodology can be used for predicting and optimizing 2f-WM signal strengths and shapes from Rb atoms (and thereby presumably also from other atoms) under a variety of pressure and temperature conditions.     

The production of Te atoms by the multiphoton dissociation of diethyltelluride

Detection of Te atoms in the ground ³P^2,1,0 states has been accomplished using two-photon laser-induced fluorescence (LIF) and “2 + 1” multiphoton ionization (MPI). Te atoms were produced by multiphoton dissociation of (C₂H₅)₂Te in the region 358–395 nm. The LIF and MPI experiments utilize selective excitation of the 6p ³P_J′←5p ³P_J″ and 7p ³P_J′←5p ³P₂ transitions by two-photon absorption. Line strength intensities for the individual J′ ← J″ fine structure pairs of the 6p ³P_J′ ← 5p ³P_J″ two-photon transitions were calculated and compared to the LIF data. Intensities of these transitions of Te atoms are compared to analogous ³P ← ³P transitions in S and O atoms.     

Photochemical decay of butynal (H3CCCCHO) in the gas phase following S1(nπ*) ← S0 excitation

After excitation into the S₁(nπ*) ← absorption, butynal decomposes into CO and propyne (H₃CCCH). Emission lifetimes, photoproduct quantum yields, and phosphorescence quantum yields were measured in the pressure range 0.25–2 Torr at room temperature using excitation energies between 26620 (0-0 transition) and 33300 cm^?1. Based on these results and on those of the propynal (HCCCHO) dynamics previously reported. a photodissociation mechanism for butynal is proposed which features dissociation from the hot S₀ ground state by a concerted reaction.     

The visible spectrum of jet-cooled CF3NO

The electronic origin of the à A″ ← X̃ A′ transition of trifluoronitrosomethane (CF₃NO) has been reassigned to the very weak feature near 717.9 nm in the fluorescence excitation spectrum of the jet-cooled molecule. The prominent torsional progression has been reinpreted and the height of the threefold torsional barier in the Ã( n,π^* ) state has been revised to 601.5 ± 10 cm⁻¹.     

Changes in excited-state dipole moments with variations in pressure of the gaseous dielectric SF6

Electrochromism data are presented for the π* ← π transition of aniline and the 3s ← n transitions of acetone and cyclopentanone when the pressure of the arc-suppressor gas (SF₆) is increased. Second dielectric virial coefficients for these excited states are estimated from the variations in the dipole-moment changes.     

Radiative and nonradiative lifetimes in excited states of Ar,Kr and Xe atoms in a neon matrix

Synchrotron radiation with its intense continuum and its excellent time structure has been exploited for time resolved luminescence spectroscopy in the solid state. By selective excitation of n = 1, n′ = 2 exciton states of Xe, Kr and Ar atoms in a neon matrix we were able to identify the emitting states involved. Lifetimes within the cascade of radiative and radiationless relaxation between excited states as well as the radiative lifetimes for transitions to the ground state have been derived from the decay curves. Energy positions and radiative lifetimes of the emitting states correspond quite well with those of the free atoms. Radiative and radiationless relaxation processes take place within the manifold of excited states of the guest atoms. The rate constants for radiationless decay confirm an energy gap law. The order of the radiationless processes reaches in some cases extremely high values. Selection rules for spin and angular momentum are essential to understand the observed radiationless transition rates.     

Effect of rotational relaxation on the intensity and polarization of fluorescence emission caused by sequential two-photo excitation

General formulas are derived for the intensity and the degree of polarization of the S_m-fluorescence emission (m ? 2) of a sample excited by the sequential two-photon excitation process (S_n ← S₁ ← S₀, n ? 2) with plane-polarized pulsed light. They show how the S_m-fluorescence intensity and anisotropy depend on the relative orientation of the relevant transition dipoles within a molecule and on the degree of rotational relaxation of molecules in the intermediate state (S₁) and in the S_m state (for the case m = n), or in the S_n → S_m process (m ≠ n).     

Ion size influence on the Ar solvation shells of M(+)-C6F6 clusters (M = Na, K, Rb, Cs)

The size-specific influence of alkali metal ions in the gradual transition from cluster rearrangement to solvation dynamics is investigated by means of molecular dynamics simulations for alkali metal cation-hexafluorobenzene systems, M(+)-C(6)F(6) (M = Na, K, Rb and Cs), surrounded by Ar atoms. To analyze such transition, different small aggregates of the M(+)-C(6)F(6)-Ar(n) (n = 1, ..., 30) type and M(+)-C(6)F(6) clusters solvated by about 500 Ar atoms are considered. The Ar-C(6)F(6) interaction contribution has been described using two different formalisms, based on the interaction decomposition in atom-bond and in atom-effective atom terms, which have been applied to study the small aggregates and to investigate the Ar solvated M(+)-C(6)F(6) clusters, respectively. The selectivity of the promoted phenomena from the M(+) ion size and their dependence from the number of Ar atoms is characterized.     

Solubility of metal atoms in helium droplets: exploring the effect of the well depth using the coinage metals Cu and Ag

We report a theoretical investigation of the solution properties of Cu and Ag atoms dissolved in He clusters. Employing our recent ab initio ground state pair potential for Me-He (Me = Ag, Cu), we simulated the species Me@He (n) (n = 2-100) by means of diffusion Monte Carlo (DMC) obtaining exact information on their energetics and the structural properties. In particular, we investigated the sensitivity of structural details on the well depth of the two interaction potentials. Whereas Ag structures the first He solvation layer similarly, to some extent, to a positive ion such as Na(+), Cu appears to require the onset of a second solvation shell for a similar dense structure to be formed despite an interaction well of 28.4 μhartree. An additional signature of the different solution behavior between Ag and Cu appears also in the dependence of the energy required to evaporate a single He atom on the size of the MeHe(n) clusters. The absorption spectrum for the (2)P ← (2)S excitation of the metals was also simulated employing the semi-classical Lax approximation to further characterize Me@He(n) (n = 2-100) using novel accurate interaction potentials between He and the lowest (2)P state of Ag and Cu in conjunction with the Diatomic-in-Molecules approach. The results indicated that Ag exciplexes should not form via a direct vertical excitation into an attractive region of the excited manifolds and that there is an interesting dependence of the shape of the Cu excitation bands on the local structure of the first solvation shell.