Volume 104, Issue 22, 08 June 1996
Index of content:

Two‐photon ionization studies of binary aromatic van der Waals clusters: Benzene...chlorobenzene and (chlorobenzene)_{2}
View Description Hide DescriptionJet‐cooled van der Waals dimers of benzene (B) and chlorobenzene (BCl) were studied by one‐color resonant two‐photon ionization through the S _{0}→S _{1} transition of the chlorobenzene. The spectra of BCl...B and (BCl)_{2} in the 0–0 band both show two main features with different spectral shifts. These two main features are attributed to two different geometries: perpendicular T‐shaped geometry and parallel displaced geometry. This assignment is supported by the relative intensity dependence on the internal temperature and our ab initio calculations at the MP2 level. The binding energies of BCl...B and (BCl)_{2} were measured near the two‐photon ionizationdissociation threshold in a reflectron time‐of‐flight mass spectrometer (RTOF/MS). Our estimated binding energies of BCl...B and BCl...BCl are 0.14±0.01 eV and 0.15±0.01 eV, respectively, consistent with their increasing dipole moment and polarizability in that order.

Determination of the molecular constants of interacting bright and dark states: Analysis of the high‐resolution infrared spectrum of the S–O stretching fundamental of thionylimide, HNSO
View Description Hide DescriptionThe S–O stretching fundamental, ν_{2}, of thionylimide has been recorded at high resolution and rotationally analyzed for the first time. This strong A‐type band with origin at 1257.5808 cm^{−1} is extensively perturbed by the 5^{1}6^{1} state at 1206.58 cm^{−1} through a‐ and b‐axis Coriolis interactions. Even though no ν_{5}+ν_{6} band transitions could be found, a complete analysis of the perturbed rotational structure has been achieved. Accurate molecular constants for the ‘‘bright’’ 2^{1} state and the ‘‘dark’’ 5^{1}6^{1} state have been obtained, using a Hamiltonian that includes both types of Coriolis interactions. Further small perturbations at high K _{ a } and J have been identified as higher‐order interactions with the ν_{3} band at much lower energy.

Ultrafast molecular dynamics controlled by pulse duration: The Na_{3} molecule
View Description Hide DescriptionLaser pulses of moderate intensities with durations of either 1.5 ps or 120 fs were employed to excite the Na_{3} molecule to its electronic B state. Using a pump–probe technique the temporal evolution of the two‐photon ionization signal could be resolved in real time. Different vibrational modes of the excited trimer are detected selectively. While the ps laser pulses yield preferential excitation of the slow pseudorotational mode with a period of 3 ps, the use of ∼10 times shorter pulses allows the trimer’s symmetric stretch mode with a 310–320 fs period for the first 5 ps to be observed. These complementary experimental results can be explained to a great extent by quantum dynamical simulations of the pump–probe experiments. The calculations are performed on three‐dimensional ab initiopotential energy and transition dipole surfaces. Thus all three vibrational degrees of freedom of the Na_{3} molecule are included in the theoretical treatment. The time‐dependent wave‐packet dynamics elucidate the effect of ultrafast state preparation on the molecular dynamics. Extensive theoretical calculations manifest the possibility of initiating the molecular dynamics dominantly in selected modes during a certain time span by variation of the pump–pulse duration.

The microwave spectrum of the NCl radical in the electronically excited (a ^{1}Δ) state
View Description Hide DescriptionThe pure rotational spectrum of the ^{14}N^{35}Cl radical in the first electronically excited a ^{1}Δ state was detected by microwave spectroscopy. The NCl radical was produced by a dc‐glow discharge of an N_{2} and Cl_{2} mixture between 175–210 K. Seven rotational transitions for υ=0 and five for υ=1, showing hyperfine structures due to the nitrogen and chlorine nuclei, were observed in the 162–404 GHz region. The rotational, centrifugal distortion, and hyperfine coupling constants including nuclear spin–rotation coupling constant of the chlorine nucleus were determined accurately by a least‐squares analysis of the measured frequencies. The equilibrium structural parameters were derived and discussed.

Matrix isolation study of the interaction of excited neon atoms with BCl_{3}: Infrared spectra of BCl^{+} _{3}, BCl^{+} _{2}, and BCl^{−} _{3}
View Description Hide DescriptionWhen a Ne:BCl_{3} sample is codeposited at approximately 5 K with a beam of neon atoms that have been excited in a microwavedischarge, the infrared spectrum of the resulting solid deposit shows a weak to moderately intense absorption of BCl_{2} and more prominent absorptions which are assigned to the ν_{3} fundamentals of BCl^{+} _{3} (D _{3h }) and of linear, centrosymmetric BCl^{+} _{2}. The boron‐ and chlorine‐isotopic structure of the spectrum is consistent with both of these assignments. Ab initio calculations support the BCl^{+} _{2} assignment. An absorption is also tentatively assigned to ν_{3}(e) of BCl^{−} _{3}. Ab initio calculations for BCl^{−} _{3} are consistent with that assignment. The processes which occur when the solid deposit is exposed to visible and ultraviolet radiation are considered.

Vibronic spectrum and structure of the trans‐bent acetylene radical anion
View Description Hide DescriptionWe report the first observation of the vibronic absorptionspectrum of the trans‐bent acetylene radical anion, produced in 3‐methylpentane matrices by γ irradiation at 77 K. Two optical absorption bands are observed at 300–360 nm and in the region λ<420 nm. The former (strong) and the latter (weak) bands correspond respectively to electronic transitions to the Ψ_{2}(B _{ u }) and Ψ_{1}(A _{ u }) excited states, specifically to the vertical transitions ‖pπ_{ y } ^{*}(C≡C)〉←‖pπ_{ y }(C≡C)〉 and ‖pπ_{ y } ^{*}(C≡C)〉←‖π_{ z }(C≡C)〉 from the ground state Ψ_{ G }(A _{ g }) having an in‐plane pseudo π* type singly occupied molecular orbital: ‖pπ_{ y } ^{*}(C≡C)〉. For the ^{12}C_{2}H_{2} ^{−} anion, these bands are accompanied by a single‐vibrational progression of 1300–1240 (±20) and ∼1150 (±60) cm^{−1}, respectively. These progressions are assigned to the (v←0) transitions of C≡C stretching modes and to H–C≡C–H bending vibrational modes by comparison with the results for ^{13}C_{2}H_{2} ^{−} and ^{12}C_{2}D_{2} ^{−} anions. The appearance of only a single‐vibrational mode is reasonably well understood, from a molecular orbital calculation, as a consequence of the above electronic transitions. The frequency of the Ψ_{2}(B _{ u }) excited anion is much smaller than the frequency of the acetylene molecule ν_{C≡C}=1974 cm^{−1} and that of the ground state of the cis‐bent anion moiety in the reported [Li^{+}...C_{2}H^{−} _{2}] complex 1655 cm^{−1}. Our results indicate that the C≡C bond is greatly weakened by an addition of an excess electron to the antibonding orbital and by electronic excitation. The vibronic structure of the anion is discussed in conjunction with the results of an electron spin resonance study.

Threshold photoionization spectra of benzyl radical: Cation vibrational states and ab initio calculations
View Description Hide DescriptionWe have measured threshold photoionization spectra of benzyl^{+}‐h _{7}, benzyl^{+}‐αd _{2}, and benzyl^{+}‐d _{7} in the ground electronic state (X̃^{+} ^{1} A _{1}) using resonant two‐photon excitation and detection of electrons by pulsed field ionization. The adiabatic ionization potentials of benzyl‐h _{7}, benzyl‐αd _{2}, and benzyl‐d _{7} are 58 468±5 cm^{−1}, 58 418±5 cm^{−1}, and 58 386±5 cm^{−1}. Excitation through a variety of vibronically mixed Ã ^{2} A _{2}–B̃ ^{2} B _{2} neutral excited states allows observation of cation vibrations of both a _{1} and b _{1} symmetries. We directly measure in‐plane fundamentals and infer the frequencies of certain out‐of‐plane fundamentals from their involvement in combinations or overtones. By comparison with harmonic frequencies from ab initio calculations, we assign 35 of 48 observed levels in the ‐h _{7} isotopomer, 15 of 22 levels in ‐αd _{2}, and 25 of 30 levels in ‐d _{7}. Ab initio calculations permit a detailed comparison of the geometry, chemical bonding, and vibrational frequencies in the benzyl anion, neutral, and cation. The anion and cation, both closed‐shell species, have remarkably similar geometries with relatively short exocyclic CC bond (1.371 Å and 1.372 Å, respectively) and with the aromatic ring compressed along the C _{2} symmetry axis. The neutral free radical has a longer exocyclic CC bond (1.413 Å) and a more nearly sixfold symmetric ring. The natural resonance theory provides bond orders and resonance‐structure weights in all three species. While no single resonance structure dominates in any of the three species, the structure with an exocyclic CC double bond is significantly more important in the anion and cation than in the neutral.

Vibronic coupling mechanism in the Ã ^{2} A _{2}–B̃ ^{2} B _{2} excited states of benzyl radical
View Description Hide DescriptionWe report two‐color resonant two‐photon ionizationspectra of internally cold benzyl‐h _{7}, benzyl‐αd _{2}, and benzyl‐d _{7} radicals in the region of the vibronically mixed Ã ^{2} A _{2}–B̃ ^{2} B _{2}excited states near 450 nm. Spectra of the corresponding 1:1 van der Waals complexes benzyl⋅Ar are reported as well. Band intensities of threshold photoionizationspectra using a variety of mixed Ã ^{2} A _{2}–B̃ ^{2} B _{2} vibronic states as intermediates provide additional new information about the mechanism of vibronic coupling. A semiquantitative coupling model based on crude adiabatic states attempts to interpret all available data from absorption, dispersed fluorescence, and pulsed field ionization (ZEKE‐PFI) spectra. The two b _{1}‐symmetry modes ν_{28} (an in‐plane skeletal deformation) and ν_{21} (an in‐plane skeletal plus CCH bending motion) couple the Ã and B̃ states most strongly. In contrast to earlier interpretations, we find that the b _{1} combination ν_{17}+ν_{36} plays a prominent role, while the b _{1} in‐plane–CH_{2} rock ν_{29} is unimportant. The dispersed fluorescence work of Selco and Carrick and of Fukushima and Obi shows clear evidence of substantial coupling of the Ã and C̃ states through the a _{1} mode ν_{13}, in accord with the semiempirical vibronic coupling calculations of Negri et al. In contrast with those calculations, our model seemingly demands no Ã–B̃ vibronic coupling matrix elements larger than 100–200 cm^{−1}. Thus the dramatic effects of Ã–B̃ vibronic coupling result primarily from the near‐degeneracy of the two excited states rather than unusually strong vibronic coupling matrix elements. Some fluorescence and PFl band intensities involving ν_{28} and ν_{21} deviate substantially from simple predictions based on products of squared mixing coefficients times Franck–Condon factors. A complete understanding of the spectra will require a quantitative account of Duschinsky mixing, which in turn requires accurate excited state vibrational modes.

Laser‐reduced fluorescence study of the carbon monoxide nd triplet Rydberg series: Experimental results and multichannel quantum defect analysis
View Description Hide DescriptionThe ndσ and ndδ (n=4...9, v=0) triplet Rydberg series of carbon monoxide have been observed for the first time and rotationally resolved with an accuracy of 0.03 cm^{−1} using a three‐step excitation scheme. A multichannel quantum defect theory (MQDT) analysis shows strong perturbations for n=5, 8, and 9 and predicts the positions of the sσ and dπ levels. There is significant mixing between the sσ and dσ states, with a mixing angle between 42 and 45 degrees, as well as some smaller interaction with the np series. The ionization limit for CO is found to be 113 027.5(3) cm^{−1}.

A study of the vibronic structure in the HeI excited photoelectron spectrum of CO_{2} involving the X ^{2}Π_{ g } and A ^{2}Π_{ u } ionic states
View Description Hide DescriptionThe HeI excited photoelectron spectrum of the CO_{2} molecule covering the X ^{2}Π_{ g } and A ^{2}Π_{ u } ionic states has been recorded at a resolution of better than 5 meV. Complex vibrational structures are resolved in both photoelectron bands. In the X ^{2}Π_{ g } state, the ν_{2} and ν_{3} modes are observed to be excited in both an odd and even numbers of quanta in addition to the ν_{1} mode, whereas for the A ^{2}Π_{ u } state the spectrum is dominated by excitations of the ν_{1} mode alone and in combinations with excitations of the ν_{2} mode in two quanta involving strong Fermi resonance. The observed spectrum has been assigned by comparison with optical spectra and with calculations of the vibrational fine structure including vibronic and spin–orbit coupling.

Rovibronic coupling in the Na_{3} B system
View Description Hide DescriptionThe rovibronic spectrum of the Na_{3} B system is computed taking into account full rovibronic coupling between different vibronic states. The rovibronic coupling matrix elements are calculated using the vibronic states following from a pseudo Jahn–Teller model for the vibronic dynamics in the B system. Comparison with results of optical–optical double resonance measurements [W. E. Ernst and S. Rakowsky, Can. J. Phys. 72, 1307 (1994)] shows good agreement. In particular, certain properties of the Coriolis splittings, so far explained by an ad hoc spin–rotation interaction, are now well understood as the consequence of rovibronic coupling between two vibronic states. A general discussion of rovibronic dynamics on electronic potential energy surfaces with three equivalent minima is presented.

Molecules interacting with short, intense laser pulses: The dynamical role of quasistationary states
View Description Hide DescriptionA theoretical framework for the description of the pulse‐induced reactiondynamics of molecules interacting with short, intense laser pulses is derived. Born–Oppenheimer adiabaticity is formulated in terms of quasistationary Floquet states; its violation and the structure of the potential mixing electronic levels, and thus effecting this violation, is discussed; it is shown that, on the basis of quasistationary states, this potential is (to a very good approximation) nonvanishing only around near degeneracies in the quasienergy spectrum where it has a simple two‐level structure. Ab initio calculations showing the effectivity of this scheme are performed for the H_{2} molecule. Particular emphasis is put on bond‐softening reactions; a new mechanism is described.

Molecular radiative transport. II. Monte‐Carlo simulation
View Description Hide DescriptionThe theory of radiative transport allows in principle the accurate calculation of the fluorescence intensity and anisotropy decays, and of the fluorescencespectrum and macroscopic quantum yield, under given conditions. However, most of the coefficients of the theoretical expressions are in general not amenable to analytic form, and even their numeric computation is quite difficult. Given the probabilistic nature of the underlying processes of absorption and emission, a Monte‐Carlo (MC) simulation built upon the basic theoretical equations is particularly well suited for the task. In this work, we discuss and carry out detailed simulations for a realistic system (rhodamine 101 in ethanol) in a finite three‐dimensional volume that reproduces a common fluorescence cell. The two usual geometries of detection are considered: front face and right angle. The MC simulation method developed allows, for the first time, the accurate calculation of the effect of radiative transport on fluorescence intensity and anisotropy decays, time‐resolved and steady‐state spectra, as well as on the values of the macroscopic quantum yield and steady‐state anisotropy. Because the spatial distribution of each generation of excited molecules can also be obtained with this method, a direct and clear picture of the spatial evolution of the excitation is also obtained.

High resolution single‐photon ionization of HBr in the spin–orbit autoionization region
View Description Hide DescriptionThe rotationally resolved photoelectron yield in the ionization of jet‐cooled HBr was measured in the energy range between the spin–orbit split ^{2}Π ionic thresholds. For single‐photon excitation, narrow‐band VUV radiation was generated by resonant frequency mixing. The spectrum is complex due to interaction of autoionizing resonances belonging to several series converging to different rotational states of the ion core. For low Rydberg orders, n≤11, a description of angular momentum coupling according to Hund’s case (c) is found to be appropriate. An assignment of higher Rydberg series in the range of 17≤n≤20 was made in Hund’s case (e) coupling scheme by comparison with the corresponding photoionization spectra of Kr, HI, and HCl.

Electron thermalization in rare gases and their mixtures
View Description Hide DescriptionThe time evolution and temperature dependence of electron energy distribution functions (EDFs) are studied in pure rare gases (He, Ne, Ar, Kr, Xe) as well as in their mixtures by using solutions of the Boltzmann equation. A clear difference between the gases having the Ramsauer–Townsend (RT) minimum in the momentum‐transfer cross section, (RT gases: Ar, Kr, and Xe), and those without the RT minimum (non‐RT gases: He and Ne) is pointed out. The influence of the position and the depth of the RT minimum on the EDF and time evolution is studied for three different initial electron energies. A formula proposed for describing thermalization time in a mixture is tested on (i) a non‐RT–non‐RT gas mixture, (ii) a RT–non‐RT mixture and (iii) a RT–RT gas mixture. The linear combination of the reciprocal thermalization times in gas mixture with the component concentrations as weighting factors is found to be valid for gases with a similar energy dependence of the momentum‐transfer cross section, σ_{ m }, and also for all rare‐gas binary mixtures if the initial electron energy is sufficiently below the RT minimum. Conspicuous deviations from the linear relationship are observed in mixtures of gases whose energy dependence of σ_{ m } (or the stopping cross section) are different, and theoretical rationales for these findings are provided.

Multiplet‐specific multichannel electron‐correlation effects in the photoionization of NO
View Description Hide DescriptionWe present partial photoionization cross sections and photoelectron asymmetry parameters for the photoionization of NO leading to the (2π)^{−1} X ^{1}Σ^{+}, (5σ)^{−1} b ^{3}Π, (5σ)^{−1} A ^{1}Π, (4σ)^{−1} c ^{3}Π, and (4σ)^{−1} B ^{1}Π states of NO^{+}. The results were obtained with multichannel multiplet‐specific interaction potentials derived from correlated target states. The resulting scattering equations were solved using the Schwinger variational method. The calculations considered excitation energies in the 10–40 eV range. It was found that selective orthogonalization eliminated spurious resonances that were encountered. We found that in the channel leading to the (2π)^{−1} X ^{1}Σ^{+} state of NO^{+}, the structure seen experimentally in the 14–17 eV region is due to two pronounced valence autoionizing states, one of which is broadened by interaction with a shape resonance. We predict the existence of a third strong σ→π* transition of ^{2}Σ^{−} symmetry in the photoabsorption cross section at approximately 14.8 eV photon energy which, due to symmetry restrictions, cannot decay by autoionization. In addition, our results indicate that the broad structure seen experimentally in the 20–40 eV region in the (2π)^{−1} channel might be due to coupling to shape resonances which occur in other ionization channels. Our predicted total cross sections and photoelectron asymmetry parameters differ from those obtained by previous theoretical approaches, all of which neglected correlation effects. The present results were found to be in good agreement with the available experimental data. We found that the existence and position of the various resonances were sensitive to the level of correlation and interchannel coupling included in the calculation. In particular, we found that the resonant enhancement in the channels leading to both the (5σ)^{−1} b ^{3}Π and (5σ)^{−1} A ^{1}Π states of NO^{+} was due to a single 5σ→σ* resonant state which decayed into both ionization channels.

Molecular dynamics study of energy transfer in binary collisions of water molecules
View Description Hide DescriptionCollisional energy transfer between two water molecules, one highly energized (reactant) and another thermally equilibrated (medium) molecule, has been studied by classical molecular dynamics simulation over a range of excitation energies and medium temperatures. The focus is on the dependence of the energy transfer efficiency on the excitation energy, the medium temperature, and the gross features as well as the details of the interaction between the molecules. High quality interaction potentials based on experimental data or quantum chemical calculations are used and the results are compared with those obtained by simpler potentials constructed from Lennard‐Jones pair potentials and point charges. The dipolar contribution to the interaction is varied and the molecules are partially or fully deuterated. The strong electrostaticinteraction is found to yield efficient energy transfer for small impact parameters but also a large cross section for watercollisions. The energy transfer efficiency is sensitive to the detailed form of the interaction. However, if somewhat lower accuracy can be accepted then simple potentials can be used. The energy transfer can be well fitted by a conditional probability density based on a statistical model of equilibration among subsets of the degrees of freedom in the colliding molecules. Rotational energy transfer is far more efficient than vibrational energy transfer.

The effect of the nature of the interaction potential on cluster reaction rates
View Description Hide DescriptionThe effect of two different interaction potentials, a two‐body and a many‐body potential, on thermal clusterreaction rates was studied for 2–13 atom nickelclusters using the classical trajectory method. The reaction rates were computed for cluster–monomer and cluster–cluster collisions at T=1200 K, using the bulk and dimer parametrized Lennard‐Jones (LJ) potentials and were compared with the rates previously obtained for these collisional events by using a more realistic many‐body tight‐binding second moment approximation (TB‐SMA) potential. For cluster–monomer collisions, close agreement exists between the reaction cross section results for dimer fitted LJ (LJD) potential and TB‐SMA potential suggesting that the cluster–monomer collisions may be dominated by pairwise interactions. The bulk fitted LJ potential (LJB) underestimates the sticking cross section results of the other two potentials for most cluster sizes. This discrepancy however appears to be due to the relatively smaller cluster binding energies obtained for this potential as a result of which a larger cross section for dissociation is observed. For cluster–cluster collisions, for most cluster sizes, no agreement exists between the reaction cross section results for the three potentials. The discrepancy between the cross section results for the LJ potentials and the TB‐SMA potential appears to lie in the difference in the scaling of cluster energy with cluster coordination for these two types of potentials (i.e., linear for LJ vs square root dependence for TB‐SMA). Some characteristics of the cross section results of both LJB and LJD potentials correlate with the relative cluster stability pattern for the LJ clusters. For TB‐SMA case, no such correlation exists, which however is consistent with the smooth and featureless size distributions observed experimentally for nickel and other transition metals. The cut‐off used in the TB‐SMA potential appears to lead to a significant underestimation of the total reaction cross section for N=13, in the case of the cluster–cluster collisions. The results of this study indicate that the rate calculations may be sensitive to both the nature and parametrization of the simulation potential depending on the temperature range considered and cluster growth process simulated.

Reaction pathways in the photodetachment of an electron from aqueous chloride: A quantum molecular dynamics study
View Description Hide DescriptionReaction and relaxation processes induced by photoexcitation of an aqueous chloride ion are studied with quantum molecular dynamics simulations. A predominant channel leading to a metastable hydrated electron‐chlorine pair is found. By means of theoretical transient and stationary absorption spectra, the solvent reorganization involved in the charge repartitioning is discussed. The dissipation of excess electron kinetic energy by surrounding water molecules plays an essential role in the equilibration of an electron‐atom pair. For this intermediate species, two competing reaction pathways are identified. One is the barrier‐impeded dissociation yielding a hydrated electron. Shape and height of the free energy barrier determined by quantum umbrella sampling point to a diffusion controlled electron photodetachment. The other channel is the geminate recombination via a nonadiabatic transition for which a self‐consistent and fully dynamical treatment of the solvent electronic polarization is found to be important. From the rate constants computed for the individual channels, a kinetic model is derived to explain time‐dependent spectral signatures and electron escape yields recently observed in photodetachment experiments on aqueous halides.

Relativistic and correlation effects on molecular properties. I. The dihalogens F_{2}, Cl_{2}, Br_{2}, I_{2}, and At_{2}
View Description Hide DescriptionA benchmark study of a number of relativistic correlation methods is presented. Bond lengths, harmonic frequencies, and dissociation energies of the molecules F_{2}, Cl_{2}, Br_{2}, I_{2}, and At_{2} are calculated at various levels of theory, using both the Schrödinger and the Dirac–Coulomb–(Gaunt) Hamiltonian.