The influence of atomic velocity distribution on optical pumping of an atomic beam

Results of previous theoretical treatments of optical pumping can be used for monovelocity atomic beams only. In this paper, we present a method to deal with the optical pumping of a thermally effused atomic beam in consideration of the influence of the velocity distribution. The results show that the single velocity approximation causes considerable errors. The errors are explained and the way to minimize them is discussed.     

Rydberg states of the K2 molecule studied by laser spectroscopy in a supersonic beam

Rydberg states of potassium dimer have been studied in a crossed laser-molecular beam experiment. The K₂ molecules were formed in a supersonic expansion and excited by low-power cw dye laser. Two different excitation schemes have been used: The first scheme uses a single mode ring dye laser to induce near resonant two-photon transitions while in the second scheme stepwise excitation with two dye lasers is used. In each case excitation of Rydberg levels was detected by monitoring the ionization signal resulting from three-photon absorption. We report a detailed study of 700 two-photon resonances between 625 nm and 650 nm. Most of these signals can be assigned to transitions from the X¹σ _g ⁺ to¹σ _g ⁺ ,¹Π _g , and^1Δ _g states, which are all enhanced by the B¹Π _u intermediate state. Accurate rotational constants are given for the populated vibrational levels of these states. By stepwise excitation of Rydberg levels via theB ¹Π _u state we identify 3 series of Rydberg states as¹Δ _g (4S+nD),¹Σ _g ⁺ (4S+nD), and¹Σ _g ⁺ (4S+nS) with principal quantum numbers 7≦n≦20. Molecular constants of these and other observed but as yet unidentified states are given; quantum defects and dissociation energies are discussed.     

Inhibition of CO oxidation on RuO2(110) by adsorbed H2O molecules

Catalytic CO oxidation on the RuO(2)(110) surface was studied at 300 K by scanning tunneling microscopy (STM), high-resolution electron-energy-loss spectroscopy (HREELS), and thermal desorption spectroscopy (TDS). Upon repeatedly exposing the surface to several 10 L of CO and O(2) at 300 K, STM shows that unreactive features accumulate with each CO and O(2) titration run. HREELS and TDS show formation of increasing amounts of H(2)O, retarded formation of O-cus atoms and incomplete removal of CO-bridge molecules during O(2) dosing, and a changing ratio of single- and double-bonded CO-bridge molecules. It is concluded that H(2)O (presumably from the residual gas) is accumulating at the Ru-cus sites thus blocking them, so that the dissociative adsorption of oxygen is prevented and the CO oxidation reaction is suppressed. Some 10% CO- bridge remains on the surface even during oxygen exposure. Consistent with this interpretation, deactivation of the surface is suppressed at 350 K, at the onset of H(2)O desorption.     

(CH3)2N(C6H4)nCN系列分子二阶非线性光学系数的理论研究

在AM1和INDO/CI方法的基础上, 用自编程序计算了(CH3)2N(C6H4)nCN(n=1→6)系列分子的二阶非线性光学系数β~i~j~k和β~μ并系统地研究了共轭链长对分子二阶非线性光学系统的影响及β~μ对激光场频率的依赖关系(色散效应)     

Electronic optical response of molecules in intense fields: comparison of TD-HF, TD-CIS, and TD-CIS(D) approaches

Time-dependent Hartree-Fock (TD-HF) and time-dependent configuration interaction (TD-CI) methods with Gaussian basis sets have been compared in modeling the response of hydrogen molecule, butadiene, and hexatriene exposed to very short, intense laser pulses (760 nm, 3 cycles). After the electric field of the pulse returns to zero, the molecular dipole continues to oscillate due to the coherent superposition of excited states resulting from the nonadiabatic excitation caused by the pulse. The Fourier transform of this residual dipole gives a measure of the nonadiabatic excitation. For low fields, only the lowest excited states are populated, and TD-CI simulations using singly excited states with and without perturbative corrections for double excitations [TD-CIS(D) and TD-CIS, respectively] are generally in good agreement with the TD-HF simulations. At higher field strengths, higher states are populated and the methods begin to differ significantly if the coefficients of the excited states become larger than approximately 0.1. The response of individual excited states does not grow linearly with intensity because of excited state to excited state transitions. Beyond a threshold in the field strength, there is a rapid increase in the population of many higher excited states, possibly signaling an approach to ionization. However, without continuum functions, the present TD-HF and TD-CI calculations cannot model ionization directly. The TD-HF and TD-CIS simulations are in good accord because the excitation energies obtained by linear response TD-HF [also known as random phase approximation (RPA)] agree very well with those obtained from singly excited configuration interaction (CIS) calculations. Because CIS excitation energies with the perturbative doubles corrections [CIS(D)] are on average lower than the CIS excitation energies, the TD-CIS(D) response is generally stronger than TD-CIS.     

Comparison of intensity distribution in the spectra of complex molecules cooled in a supersonic jet and in a crystalline matrix

Comparative investigations of intensity distribution in excitation and fluorescence spectra of 1,4-dihydroxyanthraquinone (quinizarin) and 1,4,5,8-tetrahydroxy-9,10-anthraquinone cooled in a supersonic jet and in a crystalline n-octane matrix were made. It is shown that in a jet the intensity distribution is appreciably affected by saturation effects. A special analysis of the effects has been made. It is noted that the investigation of saturation effects in jet spectra may serve as a tool for the lifetime measurements of the molecular excited states. In the absence of saturation, one has defined the constants of Franck-Condon and Herzberg-Teller interactions for the molecules under investigation. It has been established that the differences of intensity distribution in the spectra of molecules in a jet and in n-octane are mainly attributed to the Herzberg-Teller parameter variation.     

Molecular beam diagnostics with internal state selection. II. Intensity distribution of a Na/Na2 supersonic beam

The profile of an Na₂ molecular beam is measured with internal state selection via laser induced fluorescence behind narrow collimating slits (slit width 0.2 to 0.5 mm). A flexible connection between the rotatable detector and the laser as well as the photomultiplex is provided by optical fibres. It is found that for pd < 0.5 Torr cm (p is the pressure in the oven, d is the nozzle diameter) the profile is approximately of trapezoidal form independent of the internal energy of the molecules. For pd > 0.5 Torr cm excessive wings appear. The population of the wings increases with pd and with the internal energy of the molecules. It is shown that scattering processes of molecules at the centerline do not cause the wings. The appearances of the wings is correlated with the onset of dimer formation in the beam outside the nozzle. Therefore, it is assumed that a large percentage of those molecules, that are formed in the free jet in intermediate energy levels with excess kinetic energy, diffuse rapidly off the centerline. Some of them are eventually deflected towards the collimating slit. An estimate of the total number of collisions shows that the relaxation of the internal energy along an off-axis path will be less complete than along the centerline. Thus, molecules that are already present in the oven or are formed in the nozzle channel are predominantly found in the narrow central part of the beam profile. A relatively large percentage of those molecules formed in the free jet are found in the wings. The cooling of the internal degrees of freedom is less efficient for the latter.     

Molecular beam diagnostics with internal state selection: Velocity distribution and dimer formation in a supersonic Na/Na2 beam

The velocity distribution parallel and perpendicular to the molecular beam axis has been determined for molecules in well defined quantum states using TOF — via optical pumping and the Doppler-shift method. It has been found that the flow velocity as well as the speed ratio changes with the internal energy of the molecule. The flow velocity increases with increasing internal energy at low pd values (p is the pressure in the oven, d is the nozzle diameter) while the opposite is true at high pd values. The parallel speed ratio is smaller for molecules in vibrationally excited states and the perpendicular velocity distribution shows excessive tails that are more pronounced for molecules in higher lying levels. The population of individual levels has been monitored via laser induced fluorescence. It does not change monotonically with pd. The population distribution is not in thermal equilibrium and can only be approximately described by a temperature of T_υ, ≈ 150 K. On the basis of these results a simple model for the influence of the recombination of atoms on the expansion is derived: Molecules are initially neither formed in the υ = 0 vibrational level nor with high internal excitation but probably with ? 1000 cm^?1 of internal energy. The recombination leads to fast atoms and molecules. It is the incomplete deceleration of these fast particles together with an efficient quenching process for the internal energy that determines the flow velocity of molecules in individual quantum states at low pd values. At high pd values the acceleration of molecules with much internal energy is incomplete because those molecules have necessarily made only few collisions.     

Protonated clathrate cages enclosing neutral water molecules: (H+)(H2O)21 and (H+)(H2O)28

This paper describes a systematic study on the clathrate structure of (H+)(H2O)21 using tandem mass spectrometry, vibrational predissociation spectroscopy, Monte Carlo simulations, and density functional theory calculations. We produced (H+)(H2O)n from a continuous corona-discharged supersonic expansion and observed three anomalies simultaneously at the cluster temperature near 150 K, including (1) the peak at n=21 is more intense than its neighboring ions in the mass spectrum, (2) the size-dependent dissociation fractions show a distinct drop for the 21-mer, and (3) the infrared spectrum of (H+)(H2O)21 exhibits only a single feature at 3699 cm(-1), corresponding to the free-OH stretching of three-coordinated water molecules. Interestingly, the anomalies appear or disappear together with cluster temperature, indicating close correlation of these three observations. The observations, together with Monte Carlo simulations and density functional theory calculations, corroborate the notion for the formation of a distorted pentagonal dodecahedral (5(12)) cage with a H2O molecule in the cage and a H3O+ ion on the surface for this "magic number" water cluster ion. The dodecahedral cage melts at higher temperatures, as evidenced by the emergence of a free-OH stretching feature at 3717 cm(-1) for the two-coordinated water in (H+)(H2O)21 produced in a warmer molecular beam. Extension of this study to larger clusters strongly suggests that the experimentally observed isomer of (H+)(H2O)28 is most likely to consist of a distorted protonated pentakaidecahedral (5(12)6(3)) cage enclosing two neutral water molecules.     

Activation of CH4 and H2 molecules by cationic zirconium(IV) complexes: a DFT study

Reactions of methane and hydrogen molecules with [(η⁵-C₅H₅)₂ZrCH₃]⁺ and (η⁵-C₅H₅)₂ZrH₃]⁺ cations were studied using nonempirical density functional theory (DFT). In all cases, the reactions begin with the formation of agostic complexes between the substrate molecules and1 or2. Transformation of these intermediates into transition states when moving along the reaction coordinate requires only slight changes in the geometry. The dihydrogen molecule reacts with1 exothermically (−8.8 kcal mol⁻¹) and barrierlessly to form2 and CH₄. Exchange of σ-bonded ligands in the1−CH₄ system proceeds through a symmetric transition state with an activation energy of 15.0 kcal mol⁻¹. According to calculations, organometallic Zr^IV complexes are promising for activation of C−H and H−H bonds under mild conditions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2248–2254, December, 1999.     

Photodynamics of the excited states of trimethylamine in a supersonic beam

The photophysics of trimethylamine (TMA) and rare gas-TMA van der Waals molecules has been studied under supersonic beam conditions. Dual exponential fluorescence decays observed for excitation of the second excited singlet state (S₂) are attributed to a novel S₂-S₁ relaxation induced by the vibrational predissociation of van der Waals molecules.     

New vibronic bands of CH2 observed in a pulsed supersonic jet

Spectroscopic studies of the methylene radical in a supersonic expansion have resulted in the observation of previously unreported CH₂ B?¹ B₁ ← ã¹A₁, hot band transitions. These ã state levels are populated by radiative cascade following multiphoton dissociation of ketene. Multiphoton excitation of ketene is also found to produce a diffuse luminescence with a lifetime longer than 50 μs. The conclusion of Lengel and Zare are singlet methylene is not produced one-photon nitrogen laser photolysis of cold ketene is confirmed.     

Shape analysis of the velocity distribution in supersonic Ar beams: Comparison between experiment and theory

The shape parameters of the velocity distribution for beam densityF(v) = C($\text{v}\text{α}$)² exp[?($\text{v}\text{?}\text{u}\text{α}$)²]

from a supersonic expansion of Ar are determined both experimentally via time-of-light(TOF) analysis and computationally by solving the Boltzmann equation for a radial flow field via the method of moments. TOF spectra are recorded by means of the detection of metastable Ar atoms. This technique eliminates velocity discrimination in the detection process. Significance test and a sensitivity analysis for the experimental and theoretical results are included. The agreement of measured and calculated shape parameters is very good. Small-angle scattering of particles, travelling with small relative velocity along the same stream line, is still effective beyond the transition region to free molecular flow and continues to modify the distribution function. A primitive model that correlates the shape parameters c₃ and c₄ with the terminal speed ratio and describes their variation with the distance from the nozzlee is developed.     

Anomalous spatial distribution of temporal and spectral behaviour within a pulsed dye laser beam

A peculiar effect, probably overlooked in some past laser induced fluorescence (LIF) and laser enhanced ionisation (LEI) experiments, is described. Such an effect has been observed when the temporal profile of an excimer laser-pumped, pulsed dye laser beam was analysed with a microchannel plate photomultiplier detector and a fast digitising signal analyser. Substantially different shapes were found at selected spatial locations, of 1 mm size, within the same laser beam. The overall time profile of the full size beam (~6 mm) can be described by the overlapping of two limiting shapes, that is, a narrow pulse, of about 3 ns full-width-at-half-maximum (FWHM), and a much broader pulse (~7 ns FWHM). The peak intensities of the limiting pulses differ in time by ~10 ns. In addition, it is shown that the two peaks also differ in wavelength by an amount which can be as large as the width of the atomic absorption line in the atomiser. The effect, which occurs in a random manner and is easily observed only with time resolution, is extremely sensitive to the alignment of the oscillator cavity of the dye laser. This was demonstrated experimentally by comparing the laser excitation and fluorescence waveforms of gallium and lead atoms in the inductively coupled argon plasma. The recognition of such anomalous behaviour is essential for a correct interpretation of some time-resolved fluorescence and ionisation experiments.     

Polymerization of neat 2‐ethylhexyl acrylate induced by a pulsed electron beam

The bulk polymerization of 2‐ethylhexyl acrylate (2‐EHA), induced by a pulsed electron beam, was investigated with pulse radiolysis, gravimetry, and Fourier transform infrared spectroscopy. The roles of the dose rate, pulse frequency, and added acrylic acid (AA) in the polymerization of 2‐EHA were examined at ambient temperature. In the range of 12.6–71.2 Gy/pulse, the polymerization of 2‐EHA was dose‐rate‐dependent: at the same total dose, a lower dose rate yielded a higher conversion. Also, a lower pulse rate gave a higher conversion at the same total dose. The addition of up to 10 wt % AA showed no increase in the conversion of 2‐EHA at a low conversion (8 kGy), but at a higher conversion (16 kGy), a 20 wt % increase in the conversion of 2‐EHA was observed. The estimated values (1.6 ± 0.3) × 10^?3 (dm³ s)^3/2 mol^?1 s^?1/2 for k_p(G/2k_t)^1/2 and 2.6 ± 0.8 dm³ s J^?1 for 2k_tG (where k_p is the rate constant of propagation, k_t is the rate constant of bimolecular termination, and G is the yield of free radicals) were obtained at relatively low conversions. The reaction rate constant of the addition of 2‐EHA^· free radicals to the monomer was measured by pulse radiolysis and found to be 2.8 × 10² mol^?1 dm³ s^?1. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 196–203, 2003     

Rate constants for the deactivation of thermal (T = 300 k) H2 molecules by hydrogen atoms

Detailed rate constants of the H + H₂(v <10) reaction have been calculated with the aim of investigating the efficiency of deactivation processes and their dependence upon the vibrational excitation of the hydrogen molecule.