We have characterized the structures of ${\mathrm{Sn}}_n$ cations up to $n=68\mathrm{}$ using ion mobility measurements. Up to $n\sim 35,$ tin clusters track the prolate growth pattern previously found for ${\mathrm{Si}}_n$ and ${\mathrm{Ge}}_n.$ However, the detailed size-dependent variations start deviating from those observed for ${\mathrm{Si}}_n$ above $n=14\mathrm{}$ and ${\mathrm{Ge}}_n$ above $n=21.\mathrm{}$ Over the $n\sim 35–65$ size range, tin clusters gradually rearrange towards near-spherical geometries, passing through several intermediate structural families. Two or three isomers are resolved for some sizes in the $n=18\mathrm{}–49$ range. The observed geometries are independent of the He buffer gas temperature between 78 and 378 K and are not affected by collisional annealing.

Statistical fragmentation can give valuable information on the internal structure of clusters due to the strong dependence of the evaporation rate on the activation energy for the process. In particular, the abundances in an ensemble of hot clusters will reflect the relative stabilities of the individual species. We derive the relation between evaporative activation energies and relative abundances for large clusters. Quantitative criteria for the applicability of the theory are derived and the analytical results are compared with numerical simulations.

The dissociation energies of doubly charged silver cluster ions in the size range $9<~n<~25$ are measured by multiple collision induced dissociation. They are compared to the dissociation energies of singly charged clusters. To this end, the latter are used to calculate shell corrections in a macroscopic-microscopic model. Good agreement between the resulting predictions of the dissociation energies of the doubly charged systems and the experimental values is found, which indicates the strong influence of electronic effects on the stability of small silver clusters.

We present an extensive experimental and theoretical study of the energy levels of the $\overline{H}$ outer-well part of the $H\overline{H}{}^1{\Sigma }_g^{+}$ state, which is the fourth adiabatic state of ${}^1{\Sigma }_g^{+}$ symmetry in the hydrogen molecule. Experimentally, rovibrational $\overline{H}$ levels were excited in a two-step laser excitation, using wavelength tunable extreme ultraviolet radiation near 91 nm to prepare selected levels in the $B{}^1{\Sigma }_u^{+}$ state $(v=18\mathrm{}$ and 19 for ${\mathrm{H}}_2$ and $v=25\mathrm{}$–27 for ${\mathrm{D}}_2,$ $v=21\mathrm{}$–23 for HD). A second tunable laser in the range 550–735 nm was used to excite the $\overline{H}$ levels in the isotopomers ${\mathrm{H}}_2,$ ${\mathrm{D}}_2,$ and HD. A third laser at 355 nm probed the excitation of the $\overline{H}$ levels by dissociative ionization, producing ions for signal detection. For ${\mathrm{H}}_2$ 82 quantum levels were calibrated $(v=2\mathrm{}$–15, $J=0\mathrm{}$–5) and 107 levels for ${\mathrm{D}}_2\hspace{0.5em}(v=6\mathrm{}$–22, $J=0\mathrm{}$–5). These level energies are compared with ab initio calculations including adiabatic and relativistic effects. Agreement between observation and calculation is of the order of $1\hspace{0.5em}{\mathrm{cm}}^{-1}$ for ${\mathrm{H}}_2$ and $0.5\hspace{0.5em}{\mathrm{cm}}^{-1}$ for ${\mathrm{D}}_2$ throughout the rovibrational manifold. In the HD isotopomer the $\overline{H}{}^1{\Sigma }_g^{+}$ state nonadiabatically interacts with the $\overline{B}{}^1{\Sigma }_u^{+}$ state. This mixing represents an example of strong $g-u$ symmetry breaking. Apart from $\overline{H}$ levels $(v=4\mathrm{}$–19, $J=0\mathrm{}–3)$ also $\overline{B}$ levels $(v=9\mathrm{}–20,$$J=0\mathrm{}–3)$ were observed. Theoretical results for HD account for the nonadiabatic interaction with the $\overline{B}$ state, based on a new ab initio calculation of the $B^{″}\overline{B}{}^1{\Sigma }_u^{+}$ potential and the coupling with the $H\overline{H}{}^1{\Sigma }_g^{+}$ state. Except for some levels near the potential barrier also for HD the experimentally calibrated levels agree with the ab initio calculated energies within $1.5\hspace{0.5em}{\mathrm{cm}}^{-1}.$

As part of a general study of the time-dependent local density approximation (TDLDA), we report here calculations of optical activity of chiral molecules, taking as examples methyloxirane and ${\mathrm{C}}_{76}.$ The theory automatically satisfies sum rules and the Kramers-Kronig relation between circular dichroism and optical rotatory power. It reproduces the absorption strengths of the strong states quite well and qualitatively describes the chiroptical properties; quantitatively, the predictions of the circular dichroism of the lowest states are a factor of 2–4 too high. In the chiral fullerene ${\mathrm{C}}_{76}$ the TDLDA provides a consistent description of the optical absorption spectrum, the circular dichroism spectrum, and the optical rotatory power, except for an overall shift of the theoretical spectrum.

We show how the inclusive rate of decay of an atomic system by absorption of any number N of photons can be calculated at $N\mathrm{th}$ order of perturbation theory without employing the final-state (continuum) wave function. We give a computationally useful formula which involves only the response function for a system perturbed by absorption of N photons. In the case of a two-electron system, the contributions to the inclusive rate from individual channels can be separated using projection operators.

Multiphoton transitions can be reached by many routes through a continuum of virtual levels. We show that the transition probability of two-photon and multiphoton processes can be controlled by tailoring the shape of an ultrashort excitation pulse, so that the many paths leading to the final state interfere to give a desired probability amplitude. We analyze the effect of pulse shapes on N-photon absorption as well as on Raman transitions. We show theoretically that certain tailored dark pulses do not excite the system at all, while other shaped pulses induce transitions as effectively as transform limited pulses, even when their peak amplitudes are greatly reduced. These results are confirmed experimentally for two-photon transitions in atomic cesium.

The interaction of laser light of arbitrary polarization with systems of high angular momentum is considered. We show that elliptically polarized light creates an anisotropic spatial distribution of atomic and molecular angular momentum which is qualitatively different from the alignment and orientation induced by light of circular or linear polarization. Multilevel coherent population trapping within a manifold of ground-state magnetic sublevels results in a nonclassical behavior of a high-$J$ molecular rotor. The classical approximation for the angular momentum distribution is compared with the exact quantum calculations, and is shown to fail in cases of long interaction times and high intensities of the exciting light. In these limits, the quantum uncertainty defines the spatial width of the angular distribution. The applicability of the classical treatment is analyzed and found to be different in the cases of $\overrightarrow{J}J-1$ and $\overrightarrow{J}J$ transitions. A biaxial spatial orientation with two preferential axes of rotation is experimentally created in sodium atoms via coherent population trapping by elliptically polarized light. A method for producing an arbitrary orientation of atomic angular momentum by magnetic field assisted coherent population trapping is proposed.

We present an extended experimental investigation of the photoassociative ionization process involving ultracold atoms. Different steps of this collisional process can be studied in detail using two independent laser frequencies. The experimental results are compared with a simple local equilibrium theory. This work complements our previous work [Phys. Rev. Lett. 70, 3225 (1993)], showing that the main physical insights of the process can be reproduced with a semiclassical theory.

The total ionization rate for photoionization from the $3d(m=2\mathrm{})\mathrm{}$ or $5g(m=4\mathrm{})\mathrm{}$ states of atomic hydrogen is calculated for the case where the incident field is a coherent superposition of a strong laser field of 620 nm wavelength and its second or third harmonic. The variation of the rate with the relative phase of the two fields is consistent with that recently described for a one-dimensional model atom [T. Cheng, J. Liu, and S. Chen, Phys. Rev. A 59, 1451 (1999)] but is significant only at very high intensities.

The interaction of a weakly bound Rydberg electron with an electromagnetic half-cycle pulse (HCP) is described with the help of a multidimensional semiclassical treatment. This approach relates the quantum evolution of the electron to its underlying classical dynamics. The method is nonperturbative, and is valid for arbitrary spatial and temporal shapes of the applied HCP. On the basis of this approach, angle- and energy-resolved spectra resulting from the ionization of Rydberg atoms by HCP’s are analyzed. The different types of spectra obtainable in the sudden-impact approximation are characterized in terms of the appearing semiclassical scattering phenomena. Typical modifications of the spectra originating from finite-pulse effects are discussed.

Nonperturbative time-dependent calculations of the resonance-enhanced two-photon dissociation probability of ${\mathrm{H}}_2$ in two frequency laser fields from the ground $X{}^1{\Sigma }_g^{+}(v=0,j=0)$ level to the final continua of $\mathrm{GK}{}^1{\Sigma }_g^{+}$ and $I{}^1{\Pi }_g$ states have been made as functions of the laser frequencies. The two fields are taken to have linear parallel polarizations with identical sine-squared time dependences of the amplitudes. The first field of frequency ${\omega }_1$ is near resonant with the two closely spaced excited intermediate levels $B{}^1{\Sigma }_u^{+}(v=14,j=1)$ and $C{}^1{\Pi }_u^{+}(v=3,j=1),$ which are strongly coupled to each other through nonadiabatic interaction as well as by radiative Raman coupling. Thus two intermediate levels with mixed ${\Sigma }^{+}-{\Pi }^{+}$ character are created. The molecule finally dissociates through coherent excitation to a number of near-resonant discrete rovibrational bound levels of $H\overline{H}{}^1{\Sigma }_g^{+}$ and $J{}^1{\Delta }_g^{+}$ embedded into the continua of GK and I states as well as by direct transition to these continua, by absorption of a second photon of frequency ${\omega }_2.$ The nonadiabatic interactions of the bound levels of $H\overline{H}$ and J states, with the respective continuum of GK and I states, give these levels a predissociating character. The interference of the direct transition amplitudes to the continua, and those through the various overlapping predissociating resonances, gives rise to a resultant structure in the dissociation probability with the variation of ${\omega }_2.$ The bound levels used are either (a) a group of three closely spaced vibrational-rotational levels, $H\overline{H}{}^1{\Sigma }_g^{+}$ $(v=4,$ $j=0\mathrm{}$ and 2), $J{}^1{\Delta }_g(v=4,j=2);$ and (b) the next group of three closely spaced levels, $H\overline{H}{}^1{\Sigma }_g^{+}$ $(v=5,$ $j=0\mathrm{}$ and 2), $J{}^1{\Delta }_g(v=5,j=2).\mathrm{}$ Far from resonance with any predissociating level, the dissociation probability becomes equal to the value obtained by considering only the direct transitions to the continua. For different fixed values of ${\omega }_1,$ the variation of the dissociation probability against ${\omega }_2$ reflects the characteristics of the excitation of the intermediate resonances with mixed B and C character. On or near resonance, the excitation of the predissociating levels of the J electronic state plays a crucial role in determining the dissociation line shape, whereas the excitation of the predissociating levels of the $H\overline{H}$ electronic state do not affect the dissociation line shape significantly.

Saturation spectroscopy capitalizing on the presence of negative crossover signals has been exploited to measure the ratio of two Landé $g_J$ values in atomic argon. The result, $g_J{(2p}_9{)/g}_J{(1s}_5)=0.88845\mathrm{}(7),$ leads to a value for $g_J{(2p}_9)=1.3335\mathrm{}(1),$ which is 100 times more certain than its best previous determination but discrepant with existing theoretical values.

Resonant enhancement of high-order peaks in the photoelectron spectrum of argon at 800 nm is studied by numerical precision integration of the time-dependent Schrödinger equation in the single-electron approximation. It is shown that electrons in the backscattering region of the spectrum are almost exclusively due to resonances. Wave-function analysis shows that there are two types of resonant states: high-angular-momentum states that stay away from the nucleus and mainly decay by emission of low-energy electrons, and states that are located near the polarization axis. These latter states predominantly decay through violent collision with the ionic core, and are responsible for the enhancement of high-energy photoelectrons.

Results from experiments and model calculations on rotational interference in vibrational ladder climbing in the $X{}^2{\Pi }_{1/2}$ state of NO are presented. In the low-fluence limit, the population in excited vibrational states shows an oscillatory behavior as a function of the chirp of the IR laser pulse. The interference is due to the accumulated phase difference of pathways involving different intermediate rotational states. The theoretical results match the experiments on an absolute scale.

The dynamics of the vibrational excitation and dissociation of a diatomic molecule by infrared laser pulses is studied classically. The system is modeled by a Morse oscillator interacting with a classical electric field. A chirped infrared laser pulse with realistic pulse shape and time-dependent frequency is very efficient to pump the molecule to highly excited states. The addition of a second fixed-frequency laser pulse is found to drastically enhance the dissociation probabilities.

We study two-color photoionization of helium in the region of the $3s3d{}^{1}D$ autoionizing state with resonant excitation of the intermediate $2s2p{}^{1}P$ autoionizing state. We show that the corresponding time-dependent Schrödinger equation may be solved by expanding the total wave function on a few complex energy states. Perimetric coordinates are used to describe correlation effects accurately. Populations at the end of the interaction as well as photoionization probabilities are studied as a function of the laser pulse duration and both frequencies. Our calculations show that many continua play a crucial role at high intensities.

We investigate the validity of classical rescattering models for describing high-harmonic generation with coherent bichromatic driving fields. We compare the fully time-dependent solution of the one-dimensional Schrödinger equation to the predictions of the semiclassical three-step model, with particular emphasis on the return times and cutoff energies. The classical return times can be compared to the time of harmonic emission, obtained from the solution of the time-dependent Schrödinger equation with the help of a time-frequency analysis. Our strikingly coincident results indicate that also in the bichromatic case, provided the field is sufficiently intense and the two driving harmonic fields of comparable intensity, the three-step model describes the essential physical process and has a very good predictive power.

Hyperpolarized ${}^3\mathrm{He}$ gas, where the nuclear spin polarization has been increased to several tens of percent by optical pumping, can couple such a large negative resistance into an external circuit that masing can ensue. The masing threshold can be suppressed by application of a magnetic field gradient. However, edge enhancement, that is, the less effective diffusional damping of the precessing magnetization at the container walls, can lower the masing threshold. The edge enhancement of the masing is greatly modified by magnetic self-interactions of the spins.

We predict and confirm numerically the existence of localized Rydberg wave packets that travel on classical elliptical trajectories with negligible spreading. These wave packets are supported by a resonant circularly polarized microwave field, and we study them by a semiclassical action-angle representation.