Photolysis of NO2 at multiple wavelengths in the spectral region 200–205 nm

A study of the photodissociation dynamics of NO₂ in the 200–205 nm region using resonance enhanced multiphoton ionization (REMPI) in conjunction with the velocity map imaging technique is presented. We chose this region because it allowed the use of a single laser to photodissociate the NO₂ molecule and probe both the O(¹D₂) fragment using (2+1) REMPI via the 3p'¹P₁ state at 2 ×205.47 nm and the 3p'¹F₃ state at 2 ×203.5 nm, and the O(³P_J) fragments using (2+1) REMPI via the P_J states around 2 ×∼200 nm. Translational energy and angular distributions are extracted from the O(¹D) and O(³P) product images. A growth in the population of highly excited vibrational levels of the NO X(²Π) co-fragment is found as the dissociation wavelength decreases. These are compared with similar trends observed previously for other triatomic O-atom containing molecules. Detailed information on the electronic angular momentum alignment of the ¹D₂ state is obtained from analysis of the polarization sensitivity of the O(¹D) images using the two resonant intermediate states. The angular dependence of the potential energy in the exit channels is examined using long-range quadrupole-dipole and quadrupole-quadrupole interaction terms, from which molecular-frame multipole moments of the total angular momentum of the recoiling O atoms have been calculated. Comparison with the experimentally derived multipole moments is used to help provide insight into the dissociation mechanism.     

Vibrational resonances of non-rotating HCN in the excited electronic state

A multi reference internally contracted configuration interaction (MRCI) method is used to generate the potential energy function (PEF) of the excited electronic state of HCN molecule. The analytic representation of the PEF is employed to calculate complex eigenvalues (resonance positions and widths) by a discrete variable representation (DVR) of the Hamiltonian for the non-rotating (J =0) molecule. The computational method used is a variant of the filter-diagonalization technique based on a recursive polynomial expansion of the absorbing-boundary-conditions (ABC) Green operator. Reasonable agreement with existing experimental data is found. Received 27 July 1999 and Received in final form 18 October 1999     

Nonadiabatic fragmentation of excited molecular ions: An exploratory study for

The photodissociation of was studied in a one-dimensional approximation, with the aim of understanding the nonadiabatic features of the fragmentation dynamics. In the collinear arrangement of the system, the three lowest excited surfaces (states) interact via two avoided crossings outside the Franck-Condon region, and they are strongly coupled radially by nonadiabatic terms. Electronic transition probabilities for the process , with in one of the three lowest electronic states involved in the fragmentation, were calculated using the semiclassical multichannel S-matrix within the half-collision approach to photodissociation. The reliability of the semiclassical theory, for treating multichannel nonadiabatic processes was analyzed, and inelastic cross-sections for the three processes of electronically selected fragmentations were calculated. The structure found in the calculated absorption lineshapes reveals the marked influence of the nonadiabatic couplings between excited states in the fragmentation dynamics of this molecular ion. Received: 6 March 1998 / Revised: 7 March 1998 and 17 June 1998 / Accepted: 23 June 1998     

Nonresonant spontaneous bremsstrahlung by a relativistic electron scattered by a nucleus in the field of pulsed light wave

This paper presents theoretical study of nonresonant spontaneous bremsstrahlung by an electron scattered by a nucleus in the field of the pulsed light wave. The electron interaction with a Coulomb potential of a nucleus is considered in the first order of perturbation theory (the Born approximation), and the interaction with an external pulsed field is taken into account accurately. The approximation is examined when pulsewidth is considerably greater than the characteristic time of wave oscillations. The obtained differential cross-section of the process has form of a sum over partial differential cross-sections. Each of them corresponds to processes with emission (absorption) of certain number of wave photons. It is shown, that for spontaneous bremsstrahlung by an electron scattered by a nucleus in the field of a pulsed light wave the distribution of a stimulated emission-absorption probability is determined by the average value of corresponding probabilities in the case monochromatic wave.     

Resonant scattering of an electron by a muon in the field of light wave

In the frame of the Born approximation we theoretically investigate resonant scattering of an electron by a muon in the field of elliptically polarized electromagnetic wave in the general relativistic case. It is studied kinematics of a scattering of an electron in resonant region. It is derived the expressions for the amplitude and the cross-section of the resonant scattering of an electron by a muon when the invariant intensity parameter of the laser field is small (η_e ≪1). It is demonstrated that the resonant cross-section of a scattering may be several orders of magnitude higher than the cross-section of a scattering in the absence of the external field.     

Isotope dependent predissociation in the C 1 Σ + , v =0 and v =1 states of CO

Rotationally resolved spectral lines in the C - X (1, 0) band of carbon monoxide are investigated under high resolution using a coherent vacuum ultraviolet laser source, continuously tunable near 107 nm. Transition frequencies are determined by calibrating against a reference standard of iodine lines, recorded with saturation spectroscopy in the visible range, yielding an absolute accuracy of 0.003 cm^-1 in the vacuum ultraviolet. Improved molecular constants for the excited state are derived and no effects of perturbation are found at the present level of accuracy. Line broadening measurements result in information on the excited state lifetime of the C ¹ Σ ⁺ , v = 1 state for five natural isotopomers of carbon monoxide: τ( ¹² C ¹⁷ O ) = 280 ps, τ( ¹² C ¹⁸ O ) = 210 ps, τ( ¹³ C ¹⁶ O ) = 295 ps, τ( ¹³ C ¹⁷ O ) = 160 ps, and τ( ¹³ C ¹⁸ O ) = 150 ps. Within the accuracy of the present measurements no effects of J-dependent lifetimes were observed, for neither of the isotopomers. In addition direct time domain measurements of the lifetime of the C ¹ Σ ⁺ , v = 0 and v = 1 states of the main isotopomer are performed in a pump-probe experiment using a picosecond VUV-laser, yielding τ( ¹² C ¹⁶ O ) = 1780 ps for v = 0 and τ( ¹² C ¹⁶ O ) = 625 ps for v = 1. For C ¹ Σ ⁺ , v = 0 in ¹²C¹⁶O and ¹³C¹⁶O the same lifetime is found; this lifetime matches experimental values of the oscillator strength and hence supports previous results showing pure radiative decay in this state; the error margins however do not exclude some low level of predissociation. The measurements indicate that the C ¹ Σ ⁺ , v = 0 state of the ¹³C¹⁸O isotopomer is predissociated with an estimated yield of 17% (i.e. above the level of predissociation for ¹²C¹⁶O.) From the combined data predissociation yields upon excitation of the C ¹ Σ ⁺ , v = 1 state are derived, lying in the range 0.84-0.91 for the five less abundant isotopomers; for the main ¹²C¹⁶O isotopomer a strongly deviating predissociation yield of 0.65 is deduced. Received 21 December 2000 and Received in final form 26 March 2001     

Molecular ions in ultracold atomic gases: computed electronic interactions for MgH<Superscript>+</Superscript>(X<Superscript>1</Superscript>Σ<Superscript>+</Superscript>) with Rb

The electronic structures of the manifold of potential energy surfaces generated in the lower energy range by the interaction of the MgH⁺(X¹Σ⁺)  cationic molecule with Rb(²S)  neutral atom are obtained over a broad range of Jacobi coordinates from strongly correlated ab initio calculations which use a Multireference (MR) wavefunction within a Complete Active Space (CAS) approach. The relative features of the lowest five surfaces are analyzed in terms of possible collisional outcomes when employed to model the ultracold dynamics of ionic molecular partners.     

Optical potential discrete variable representation method in the adiabatic representation

The optical potential discrete variable representation method (OP-DVR) has been applied recently to calculate resonances in the framework of the diabatic representation [J. Chem. Phys. 101, 7580 (1994)]. This method is based on the conjoint use of the discrete variable representation (DVR) method and the properties of a complex absorbing potential (CAP). The OP-DVR method is the DVR version of the CAP stabilization method initially proposed by Jolicard and Austin [Chem. Phys. Lett. 121, 106 (1985)]. In the present study, we show that this efficient and accurate method can also be applied within the adiabatic representation since it allows one to overcome in a simple way, numerical difficulties associated with the first derivative operator which appears in the expression of non adiabatic couplings. Within the OP-DVR method, the choice of the representation (diabatic or adiabatic) is governed by physical arguments and by the fact that the potentials and the couplings are known in one or the other of these two representations. In the case where the potentials and the couplings are obtained in the adiabatic representation, we show in this paper that the transformation into the diabatic framework is not necessary. We demonstrate that the discrete variable representation can be a simple and an efficient way to deal with the adiabatic representation. Received: 30 April 1998 / Revised: 29 September 1998 /Accepted: 21 October 1998     

Photofragment space distribution in the photodissociation of NaI in the spectral range 315–370 nm: the role of molecular axis rotation

The first determination of translational anisotropy parameters β in the photodissociation of NaI molecules in the spectral range 315–370 nm is reported. The anisotropy parameters were determined by the analysis of Doppler resolved absorption profiles of Na(²S_1/2) atoms produced in the photodissociation of NaI by linearly polarized light. The profiles were recorded for two orientations of the photolysis light: parallel and perpendicular to the direction of the probe beam. The value of the parameter β was obtained from a simultaneous fit of the profiles. The role of the rotation of the parent molecules on the branching ratio between parallel and perpendicular transitions in NaI during dissociation is discussed.     

Photodissociation dynamics of CF3I investigated by two-color femtosecond laser pulses

We report an experimental study of the multiphoton dissociation dynamics of CF₃I performed on a home-built femtosecond laser pump-probe system, with time-of-flight mass spectrometer. The first repulsive A band and the 5pπ³7sσ υ₂=1 Rydberg state of CF₃I were accessed by one- and two-photon transitions at 267 nm, respectively, with the latter two-photon absorption followed by a further two-photon probe transition at 401 nm to the state of the parent ion. The observed signals from the CF₃ ⁺ and I⁺ fragments show similar multi-component exponential decay patterns but the former is 4 times stronger than the latter. However, the parent CF₃I⁺ signal was observed to evolve in a very different manner, decreasing sharply when probed in the first 289 fs following excitation and subsequently rising again after 860 fs to a constant level below that measured at negative pump-probe delay times when the pump and probe pulses exchange roles. This dip observed in the parent ion profile, is very different from that previously reported at shorter pump wavelengths of 264 nm or 265 nm, and is interpreted as the competition between two different ionization channels. One from the vibrationally excited υ₂=1 of the irradiated Rydberg state and the other from the dissociative vibrational origin of the same electronic state which is populated by internal vibrational relaxation.     

Echo spectroscopy of atomic dynamics in a Gaussian trap via phase imprints

We present a simultaneous analysis of the X¹ Σ ⁺ and the a³ Σ ⁺ electronic ground states of the NaK molecule. Excitation of the [B ¹ Π, c³ Σ ⁺]-system made it possible to record fluorescence to rovibrational levels of both ground states simultaneously with a Fourier-transform spectrometer. For the first time high lying levels in the triplet state with v = 17 and v = 18 were seen, the highest possible v = 19 for ²³Na-³⁹K was not observed. The joint evaluation of the retrieved data leads to accurate potential energy curves, that describe the experimental data within their experimental uncertainty of 0.005 cm⁻¹. Cold collision properties like scattering length and Feshbach resonance positions are calculated with these potentials and compared to other predictions.     

Two body loss rate in a magneto-optical trap of metastable He

We have measured the two body loss rate in a magneto-optical trap containing triplet metastable He atoms. We find a rate constant cm³/s at a -8 MHz detuning, with an uncertainty of a factor 2. This measurement is in disagreement with a recent experiment which measures the absolute, ion-producing collision rate, but agrees with several other published measurements. Received 20 April 1999 and Received in final form 12 July 1999     

Cold inelastic collisions between lithium and cesium in a two-species magneto-optical trap

We investigate collisional properties of lithium and cesium which are simultaneously confined in a combined magneto-optical trap. Trap-loss collisions between the two species are comprehensively studied. Different inelastic collision channels are identified, and inter-species rate coefficients as well as cross-sections are determined. It is found that loss rates are independent of the optical excitation of Li, as a consequence of the repulsive Li^*-Cs interaction. Li and Cs loss by inelastic inter-species collisions can completely be attributed to processes involving optically excited cesium (fine-structure changing collisions and radiative escape). By lowering the trap depth for Li, an additional loss channel of Li is observed which results from ground-state Li-Cs collisions changing the hyperfine state of cesium. Received 28 December 1998 and Received in final form 16 February 1999     

Orientation of Cs 6p 2P3/2 atomic photofragments in a magnetic field: interference in the photodissociation of diatomic molecules

In a magnetic field, Cs₂ molecules were excited from the ground X¹Σ⁺_g(v_X=0,J_X=55) level to the D ¹Σ⁺_u(v=46,J=54) level by dissociation laser light linearly polarized parallel to the field, for which the magnetic sublevels were degenerated and thus all the transitions between them were simultaneously stimulated. Probe laser light excited the dissociated Cs 6p²P_3/2 atomic fragments to 6p²D_3/2 level and the resultant 6p²P_1/2 - 6d²D_3/2 emission was detected as the function of the wavelength of the probe light. The populations of the 6p²P_3/2,mj magnetic sublevels were determined from the relative strengths of the 6p²P_3/2,mj - 6d²D_3/2,m'j transitions induced by the probe light. Non-zero orientation O₀ was found in the ensemble of dissociated Cs 6p²P_3/2 atomic fragments. The orientation O₀ increased as the magnetic field strength increased. It was demonstrated both experimentally and theoretically that the orientation O₀ was induced through the interference in the excitation and dissociation paths in the presence of an external magnetic field, even when all degenerated transitions between the magnetic sublevels of the molecules are simultaneously excited by the light linearly polarized parallel to the field.     

Structure and dynamics of cationic van-der-Waals clusters

Ar_nHCl⁺ van-der-Waals clusters for n = 1–13 are investigated with the “minimal diatomics-in-molecules (DIM) model” using ab-initio input data obtained from multi-reference configuration-interaction calculations plus subsequent projection onto valence-bond wavefunctions. The results for the complexes with n = 1–3 are checked against ab-initio calculations at the coupled-cluster (CCSD) level with the same one-electron atomic basis set as for the input data generation (aug-cc-pVTZ from Dunning). In addition to the electronic ground state, the first excited state for the triatomic complex (n = 1) is also studied. The results from the DIM model are shown to be in fair agreement with those from advanced conventional ab-initio calculations, although there are differences in detail. The comparison justifies the extension of the DIM approach to n > 3. Systematic analysis of the local minima of the multi-dimensional potential-energy surfaces (PESs), carried out with the combined method described in part I (Monte-Carlo sampling plus subsequent steepest-descent optimization), reveals simple building-up regularities for the most stable structures (i.e. those corresponding to the global PES minimum) at each n: apart from always having a nearly linear (Ar–H–Cl)⁺ fragment as core, the aggregates show little or no symmetry. Secondary local minima are also determined and their structures interpreted. The PESs for the low-lying excited states reveal a much more complicated topography compared to the Ar_nH⁺ clusters allowing a variety of photo-processes. The energy level sequence of the first five excited electronic states and the stability of the clusters in these states is studied as a function of the cluster size n.     

Direct measurement of fine structure changing collisional losses in cold trapped 85Rb

Using a technique that consists in ionizing atoms out of the 5P _1/2 fragments originated in the cold collision process, we have measured the contribution of the fine structure changing collision (FS) to the total trap loss rate of cold ⁸⁵Rb. Our results show that FS contribution is responsible for about 4% of the total trap loss. This result should stimulate new theoretical discussions involving exoergic cold collisions. Received 26 October 1998 and Received in final form 2 February 1999     

Predissociation resonances in CO and IBr. Smooth exterior scaling combined with the discrete variable representation

Smooth exterior scaling (SES) and the discrete variable representation (DVR) are combined to accurately compute predissociation resonances of a bound state non-adiabatically coupled to a dissociative state. For the CO( predissociation interaction good agreement is found with approaches based on optical potentials and complex scaling. The comparison is done both in the diabatic and the adiabatic representation. The effect of the coupling strength in the IBr predissociation interaction and the transition from the diabatic to the adiabatic picture was studied by computing resonances for coupling strengths from up to . The transition from weak (diabatic) to strong (adiabatic) coupling was clearly seen. The intermediate case leads to a complicated resonance distribution. Comparison was made with recent studies using pump-probe spectroscopy [M. Shapiro, M.J.J. Vrakking, A. Stolow, J. Chem. Phys. 110, 2465 (1999)]. It was found that the overall features of the experiment could be explained from the resonance distribution, but for a detailed comparison more accurate potential energy surfaces and couplings are needed. Received 12 July 1999 and Received in final form 6 December 1999     

Description of electrons correlation in1,3S-states of negative ion of positronium

The energy of ground^1,3S-states of negative positronium ion in the hyperspherical coordinates method has been calculated in Born-Oppenheimer and adiabatic approximations accounting the contribution of angle and radial correlation. Quasi-crossing points for autodetaching adiabatic potentials were revealed, which forms a structure in nonadiabatic potential. The received energy values have been compared with results of calculations performed using other methods. It was shown that in order to receive energy values with accuracy of 10⁻⁴ a.u. it is necessary to include more than 70 elements into the basis for determination of adiabatic potentials or to use Padé approximation on the basis dimension. Presented at the 11th Colloquium “Quantum Groups and Integrable Systems”, Prague, 20–22 June 2002. The work is supported by the Slovak Agency for Science, Grant 2/7157/20.