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We present a sequential molecular dynamics/quantum mechanics (MD/QM) study and steady-state spectroscopy measurements of the nanostar dendrimer (a phenylene−ethynylene dendrimer attached to a ethynylperylene chromophore) to determine the temperature dependence of the electronic absorption process. We studied the nanostar as separate units and performed MD simulations for each chromophore at 10 and 300 K to study the effects of the temperature on the structures. The absorption spectrum of the nanostar, at 10 and 300 K, was computed using an ensemble of 8000 structures for each chromophore. Quantum mechanical (QM) ZINDO/S calculations were performed for each conformation in the ensemble, including 16 excited states for a total of 128 000 excitation energies, and the intensity was scaled linearly with the number of conjugated units. Our calculations and experimental spectra measured for the individual chromophores and the nanostar are in good agreement. We found that for each system, the spectral features ar...

We present results on the energetics and dynamics of Au atoms interacting with alkanethiol monolayers using molecular dynamics in conjunction with quantum-mechanical calculations. Using a classical force field calibrated by first-principle calculations, we have found that under certain physical conditions, gold atoms can penetrate the molecular monolayer to join the underlying Au(111) surface. We have investigated the dependence of surface coverage and temperature, and have found that our results agree well with available experimental observations. The electronic structure of the system, the potential energy as a function of penetration depth, and the charge transfer at the interface between the molecules and the Au(111) surface are analyzed to determine the potential implications for experiments.

We investigate the UV-induced desorption of O2 from a reduced TiO2 rutile surface. The desorption mechanism considered is a direct, optical excitation of the adsorbate-substrate complex. The low-lying excited electronic states of the adsorbate-surface system and the transition dipole moments are calculated with a cluster model. We also estimate the effects of nonadiabatic coupling on the desorption yield. As a consequence of the localized nature of the adsorption process, ground state properties calculated with the cluster model agree well with previous results obtained with a slab model. To reduce the size of the calculation, we use optimal orbitals for each of the interacting electronic states, followed by transformation to biorthonormal orbital sets. Comparison with experimental photodesorption cross-section data shows excellent agreement for photon energies close to the vertical transition energy. At higher energies, we speculate that substrate-mediated electronic excitation mechanisms must be include...

We present a theoretical analysis of O2 adsorption on a reduced TiO2 (110) rutile surface, based on periodic ab initio Hartree–Fock calculations. Three different orientational approaches, three different spin symmetries, and two different adsorption sites are considered. We also consider the possibility that the surface can absorb more than one oxygen molecule. Positions of the surface ions, oxygen ions belonging to the third and fourth layers of the slab, and the bond lengths of the O2 and O2-substrate distances are optimized. Adsorption energies, admolecule-substrate bond lengths, spin densities and Mulliken charges are analyzed. The model is tested by comparing spin densities and relaxation parameters obtained for the reduced TiO2 (110) system to previous theoretical results. Finally, we discuss the relationship of our results to experimental observations of thermal desorption rates at low temperatures (100–600 K).

We present an analysis of the A0(+)(6(3)P(1))--X0(+)(6(1)S(0)) bound-bound and bound-free fluorescence spectrum, and of the A0(+)(6(3)P(1))--X0(+)(6(1)S(0)) and B1(6(3)P(1))--X0(+)(6(1)S(0)) bound-bound excitation spectrum of the HgKr van der Waals molecule. The A--X fluorescence spectrum, which was observed for the first time, as well as the excitation spectra were recorded using a pulsed supersonic molecular beam crossed with a pulsed dye laser beam. An analysis of the A(v')--X(v"), B(v')--X(v"), and A(v'=8)--X(v") bound-bound bands indicates that a Morse function combined with a long-range approximation represents the interatomic potential energy curve of the A, B, and X states below the dissociation limit. In the simulation of the A(v'=8)--X bound-free spectrum the Morse, Lennard-Jones (n-6), and Maitland-Smith (n(0), n(1)) functions were tested, and the Maitland-Smith (11.39, 10.50) potential was found to be a good representation of the repulsive part of the X-state PE curve above the dissociation limit, over the internuclear separation range R=2.85-3.55 Å. The spectroscopic characteristics for the A, B, and X states obtained in this work are compared with other available experimental and theoretical results. Copyright 2001 Academic Press.

We investigate two-photon, selective excitation of diatomic molecules with intense, ultrafast laser pulses. The method involves transfer of a vibrational population between two electronic states by shaping of light-induced potentials (LIPs). Creation and control of the LIPs is accomplished by choosing pairs of transform-limited pulses with proper frequency detunings and time delays. Depending on the sequence of pulses (intuitive or counter-intuitive) and on the sign of the detuning (below or above the first transition) four schemes are possible for population transfer by LIP shaping. We develop a simple analytic model to predict the optimal laser pulses, and to model the adiabatic dynamics in the different schemes. Based on a harmonic, three-state model of the sodium dimer we demonstrate numerically that all four schemes can lead to efficient, selective population transfer. A careful analysis of the underlying physical mechanisms reveals the varying roles played by the adiabatic and diabatic crossings of the LIPs. The detailed mechanisms influence the robustness and experimental applicability of the schemes.

We present a general theory of adiabatic rapid passage (ARP) with intense, linearly chirped laser pulses. For pulses with a Gaussian profile and a fixed bandwidth, we derive a rigorous formula for the maximum temporal chirp rate that can be sustained by the pulse. A modified Landau-Zener formula displays clearly the relationships among the pulse parameters. This formula is used to derive the optimal conditions for efficient, robust population transfer. As illustrations of the theory, we present results for two- and four-level systems, and selective vibronic excitation in the I2 molecule. We demonstrate that population transfer with chirped pulses is more robust and more selective than population transfer with transform-limited pulses.

Triatomic van der Waals complexes of mercury have been observed in a supersonic expansion beam crossed with a laser beam to produce excitation and fluorescence spectra. The expansion beam consisting of mercury and a (noble) carrier gas (RG) was excited in three separate spectral regions: in the vicinity of the Hg 2537 A intercombination line, near the (forbidden) Hg 6(3)P2-- 6(1)S0 atomic transition (2200-2290 A), and in the region of the Hg2 G0(+)u-- X0(+)g transition (1975-2080 A). The resulting spectra, which were recorded in the spectral range 1900-6000 A, contained, in addition to the known Hg, Hg2, and HgRG components, new absorption and fluorescence bands that are ascribed to Hg2RG and Hg3 complexes. Copyright 1998 Academic Press. Copyright 1998Academic Press

Inherently quantum mechanical, transient nanostructures with dynamics on a picosecond time scale can be produced by controlling the quantum dynamics of Rydberg electrons. Ultrashort, tailored light pulses are used to build structures in position and/or momentum space from the wave function of an electron. The optimal laser pulses required to make these structures can be computed and synthesized in the laboratory. As an example, we present a structure consisting of five peaks which acts as a time-dependent grating and can be detected via ultrafast extreme ultraviolet diffraction. {copyright} {ital 1997} {ital The American Physical Society}

We perform high-level, quantum molecular dynamics simulations of proton transfer in the protonated water dimer, H5O2+. The electronic structure of the system is calculated concurrently with the nuclear motion using Born–Oppenheimer molecular dynamics plus density functional theory. Performing the calculations at finite (thermal) temperatures allows us to observe and quantify such effects as the broadening of the electronic density of states, the thermal splitting of degenerate states, the shift of the highest occupied molecular orbital, the thermal expansion of the dipole moment, and the thermal shift, coupling and broadening of the vibrational density of states. At two of the temperatures considered (225 K and 360 K), we find that H5O2+ exists in a dynamical equilibrium state in which the proton oscillates between two water molecules. The characteristic frequencies of the proton motion are very sensitive to temperature. At 40 K and 225 K, strong peaks are identified in the vibrational spectrum correspond...

The G 0 u + (6 1 P 1 )– X 0 G + excitation and fluorescence spectra of Hg 2 van der Waals molecules, produced in a pulsed free-jet supersonic expansion beam crossed with a pulsed dye-laser beam, were studied using Ar as the carrier gas. A well-resolved vibrational structure of the G 0 u + ← X 0 g + excitation spectrum was recorded, as well as the isotopic structures of the vibrational components. A Condon internal diffraction pattern in the G 0 u + → X 0 g + fluorescence band, emitted upon the selective excitation of the v′ = 39 vibrational component, was also observed and confirmed the assignments derived from the analysis of the isotopic structure. Analyses of the spectra yielded molecular constants for both states, as well as information on the ground state potential energy curve. The results are compared with those obtained from other experiments and from theoretical calculations.

Recent experiments by Herek, Materny and Zewail [Chem. Phys. Lett. 1994, 228, 15] have demonstrated that the timing between two transform-limited, ultrafast laser pulses can be used to control the branching ratio of Na* (electronically excited atomic sodium) to Na in the photodissociation of NaI. In this work, we theoretically show that, by varying the linear chirp of the first pulse without changing its power spectrum or field strength versus time, the Na* to Na branching ratio can be controlled over a large range with a fixed interpulse delay time and a fixed form of the second pulse. Theory predicts that at 0 K the branching ratio can be varied by a factor of 3, while at high temperatures (1000 K), the factor drops to approximately 1.2 due to the effect of the wide distribution of initial states. Experimental results at 1000 K are presented and are found to be consistent with theory. Several possible experimental methods are discussed to overcome the effects of the thermal distribution of initial states.

We analyse the transition-state spectroscopy of the FH 2 - photodetachment process using the methods of spectral quantization and spectral control. The transition-state spectrum for the collinear reaction is assigned by extracting time-independent states corresponding to specific spectral peaks. It is shown that peaks with very low intensity in the original photodetachment spectrum can be greatly enhanced by controlling the initial state of the anion. The electric field required to create the non-stationary initial state is designed using a second round of control.

Isotopically pure 202 Hg vapor, contained in a quartz cell, was irradiated with the 2572.5-A output from a frequency-doubled Ar + laser, which produced Hg 2 A 0 g ± excimer molecules by photoassociation followed by collisional relaxation. Simultaneously applied probe radiation from a ring laser, scanned over the range 4150–4400 A, gave rise to the H1 u ← A 0 g ± v′ = 1 ← v″ = 0 excitation bands consisting of about 490 rotational components. The bands were subjected to a rotational analysis which yielded spectroscopic constants that are compared with some previous data.

Molecular fluorescence of mercury has been known for over 60 years, and some of the important early experiments were carried out and interpreted in the 1930s at the University of Warsaw in Poland. It was not until the mid 1970s, that the continuum bands, emitted from excited mercury vapour, were properly assigned to the lowest electronic states of the Hg2 and Hg3 molecules. Comprehensive and quantitative information on the higher states of the Hg2 excimer has only become available during the past 15 years, largely as the result of flash photolysis experiments carried out in Cambridge, and our own studies of laser-induced fluorescence with time-resolution. We now report on our recent experiments, in which we used pump-and-probe methods to produce rotationally resolved excitation spectra of monoisotopic (202Hg)2. The rotational analyses of the observed structures yielded very accurate values of spectroscopic constants, and permitted some meaningful comparisons with "ab-initio" calculations of potential-energy diagrams.

Isotopically pure 202 Hg vapor, contained in a quartz cell, was irradiated with the 2572.5 A output from a frequency-doubled Ar + laser, which produced Hg 2 A0 g ± excimer molecules by photoassociation followed by collisional relaxation. Simultaneously applied probe radiation from a ring laser, scanned over the range 4150–4650 A, gave rise to the H1 u ← A0 g + and H1 u ← A0 g − excitation spectra consisting of several hundred rotational components. Two previously unobserved rovibronic bands (one in each spectrum) were subjected to a rotational analysis which yielded spectroscopic constants that are compared with some previously reported values.

Fluorescence and excitation spectra arising from transitions between excited states of the HgCd exciplex molecule were produced in a mixture of Hg and Cd vapors following pump-and-probe laser excitation. The vapor mixture, contained in a sealed quartz cell, was irradiated by pulsed pump (3261 \AA{}) and probe (4300\char21{}4750 \AA{}) laser beams, delayed by 450 ns relative to one another. The fluorescence bands were recorded with a monochromator, a photomultiplier, and a digitizing oscilloscope controlled by a microcomputer. In addition to the previously reported and analyzed ``blue-band'' fluorescence that can be excited with a single laser, we observed short-lived and structured fluorescence bands as well as the corresponding excitation bands in the blue spectral region, which we assigned to transitions between specific states of HgCd. An analysis of the bands yielded the vibrational constants for some of the states, as well as their relative internuclear equilibrium separations. These results provide an initial basis for the draft of a partial potential-energy diagram consisting of spin-orbit states correlated to the 5 $^{3}$P manifold in Cd.

Excitation and fluorescence spectra of Hg2 were produced in a pulsed supersonic molecular beam crossed with a pulsed laser beam. Well-resolved structures arising from [Formula: see text] and [Formula: see text] vibronic transitions were analyzed and yielded spectroscopic constants for the excited and ground states of the molecule.

Excitation spectra of HgNe and HgAr van der Waals molecules were produced in a pulsed supersonic molecular beam crossed with a pulsed dye-laser beam. Bands arising from A0 + ← X0 + and B1 ← X0 + vibronic transitions were analyzed, yielding spectroscopic constants for HgNe and HgAr. A0 + → X0 + and B1 → X0 + fluorescence spectra of HgAr were also recorded, consisting of Condon internal diffraction patterns arising from bound-free decays of various vibronic states, together with vibrational components produced by bound-bound transitions. Term analyses of the spectra combined with computer-modeling calculations produced improved values of spectroscopic constants.

Applying a spectral quantization method, we find the positions and widths of 32 transition state resonances in the three‐dimensional reaction H+H2 with J=0. The assignment of many of the resonances appears to follow asymmetric stretch and bend progressions for a linear triatomic molecule.

Transitions between the Zeeman substates of Cs 6 $^{2}\mathrm{P}_{1/2}$ and Rb 5 $^{2}$${\mathit{P}}_{1/2}$ atoms, induced in collisions with He atoms, have been investigated experimentally by methods of atomic fluorescence spectroscopy. The collision process has also been studied theoretically with time-dependent perturbation theory. Cs or Rb vapor contained in a quartz cell together with helium at low pressure was selectively excited by pulsed dye-laser radiation to the $^{2}$${\mathit{P}}_{1/2,\mathrm{\ensuremath{-}}1/2}$ Zeeman substate in a magnetic field ranging from 1.5 T to 7 T. The fluorescence spectrum, consisting of components emitted from the directly excited state and from the collisionally populated state, was resolved with a Fabry-P\'erot interferometer. Measurements of the relative intensities of the fluorescence components yielded the Zeeman mixing cross section for Cs-He and Rb-He collisions. It was found that the Cs 6 $^{2}$${\mathit{P}}_{1/2}$ cross section varied approximately as ${\mathit{B}}^{2}$ in the range 0B0.76 T, showed a much weaker field dependence in the range from 0.76 T to 5 T, where it peaked, and declined slightly at Bg5 T. The Rb 5 $^{2}$${\mathit{P}}_{1/2}$ cross section did not exhibit any variation with B. A theoretical calculation of the Cs cross section carried out using time-dependent perturbation theory explains the magnetic-field enhancement in terms of the minute magnetic-field-induced mixing of the ${\mathit{P}}_{1/2}$ and ${\mathit{P}}_{3/2}$ states and the resulting breaking of time-reversal symmetry. The results of the calculation are in good agreement with the experimental results at fields in the range 0B5 T.

The 0 + u (6 3 P 1 )←X0 + g excitation spectrum of Hg 2 van der Waals molecules, produced in a pulsed free-jet supersonic expansion beam crossed with a pulsed dye-laser beam, was studied using He as a carrier gas. A well-resolved structure, arising from transitions between the vibrational levels of the lower and upper states, was recorded at various beam conditions. An analysis of the spectrum yielded the vibrational constants, dissociation energies, and the difference between the equilibrium internuclear separations for the molecule in the two electronic states.

Pump and probe methods of laser spectroscopy were employed to excite several high-lying spin—orbit states of the HgZn exciplex. Their decays to lower bound vibronic states produced fluorescence spectra which were recorded with a monochromator in the region 4100–5200 A, and scans of the probe dye laser produced the corresponding excitation spectra. Six new spectra were recorded and analyzed, yielding molecular constants for the various states. The new data, together with a more recent version of the PE diagram, also led to improved assignments of some previously reported spectra.

Isotopically pure 202Hg vapor, contained in a quartz cell, was irradiated with frequency-doubled 5140 A output from an Ar+ laser, which caused the formation of Hg2A0±g excimer molecules by photoassociation. Simultaneous probe radiation from a ring dye laser scanned over a range 5400–6000 A, produced the G0+u ← A0+g excitation spectrum which consists of several hundred well-resolved rotational components that have not been previously observed. Two vibronic bands were subjected to a rotational analysis which yielded spectroscopic constants that are compared with some previously reported values.

5 3 P fine-structure mixing in Cd, induced in collisions in ground-state Hg atoms, was investigates by methods of LlF with time resolution. A quartz vapor-cell containing an HgCd mixture was irradiated with consecutive pump and probe laser pulses. The pump pulses (326.1 nm) produced a population of Cd 5 3 P 1 atoms whose collisions with Hg atoms caused the distribution of population among the 3 P fs states. These were probed with laser radiation exciting the 6 3 S 1 state which emitted fluorescence whose relative intensity was recorded in relation to the pum—probe delay, reflecting the time evolution of the respective 3 P J populations. The experiments which were carried out over a range of temperatures and CdHg densities, yielded the cross sections for the fs mixing collisions and the rate constant for the HgCd exciplex formation.

The results of an experimental study of the thermal dependence of laser pulse energy and of the maximum and minimum delay times between the dissociation and pumping discharge pulses are reported. (AIP)

The doubly-excited 3Σ+u (4 3P, 4 3P) state of Zn2 was populated using pump and probe methods with time resolution. Structured excitation and fluorescence spectra were observed in the 300–330 nm region and are ascribed to 3Σ+u (4 3P, 4 3P)↔3Πg(4 3P) vibronic transitions, with partial resolution of the fine structure. An analysis of the spectra yielded the vibrational frequencies, relative energy, and relative equilibrium internuclear separations for the 3Σ+u and 3Πg states. The Zn2 excitation and fluorescence spectra contained a number of atomic lines whose origin was probed in a subsidiary experiment.

The pump-and-probe method of laser spectroscopy in conjunction with a jet-expansion molecular beam was used to record the excitation spectrum arising from E1(6 3 S 1 ←AO + (5 3 P 1 ) vibronic transitions in the CdAr excimer. Two lasers were employed for the two-step excitation

We use a nonperturbative method to solve the time-dependent Schr\"odinger equation for an electronic state of an atom subject to a very intense (g${10}^{13}$ W/${\mathrm{cm}}^{2}$) laser. The oscillating, time-dependent dipole induced by the laser serves as a source for the photoemission. Calculations of single-atom photospectra reveal peaks at the odd harmonics of the incident laser field superimposed on a broad continuous background. We discuss a series of calculations for the hydrogen atom and a short-range Yukawa potential containing a variable number of bound states. At intensities up to a few times ${10}^{13}$ W/${\mathrm{cm}}^{2}$ at 1064 nm, we achieve stable, converged spectra that agree very well with previously published results. As the intensity increases to ${10}^{14}$ W/${\mathrm{cm}}^{2}$, the ionization rate increases to about 1% of the laser frequency, and converged results become extremely difficult to obtain, even for impractically large integration volumes. These difficulties are caused by a rising background due to electron density far from the nucleus and the increasing importance of the interaction of the wave function with the edges of the grid. We discuss the implications of our findings for calculations at high intensity and suggest alternative ways to calculate harmonic emission rates in the strong-ionization regime.

Atoms subject to intense laser fields are capable of absorbing many more photons than the minimum number needed to ionize. This leads to two striking nonlinear phenomena, above-threshold ionization, and high-order harmonic generation. We have used time-dependent methods to calculate both these processes nonperturbatively for realistic, three-dimensional atoms. This allows us to clarify the relationships between photon and electron emission rates in the strong field regime.

We present three‐dimensional, time‐dependent quantum‐mechanical calculations of the dynamics of the dissociation of H3 Rydberg states at total energies up to 6 eV. The method used in this work employs a Chebychev propagator in time, and computes the kinetic‐energy operators in the discrete variable representation. We calculate the total dissociation cross section, as well as partial vibrational and rotational cross sections, and compare our results to previous two‐dimensional calculations and to experiment. The results display clear three‐dimensional effects, and indicate the importance of including both sheets of the H3 ground potential‐energy surface in the dynamics.

Excitation spectra of the CdKr excimer were produced in a molecular beam employing free-jet supersonic expansion, crossed with a pulsed dye-laser beam. Bands arising from B1\ensuremath{\leftarrow}X${0}^{+}$ and A${0}^{+}$\ensuremath{\leftarrow}X${0}^{+}$ vibronic transitions were analyzed, yielding spectroscopic constants for the three states. Computer simulations of the bands yielded relative values of ${\mathit{r}}_{\mathit{e}}$, the equilibrium internuclear separations. Potential-energy curves constructed on the basis of the data and of the Morse potential provided a useful comparison with theoretically calculated curves.

We present calculations of ionization rates, trapping probabilities, and electron densities for a three-dimensional hydrogen atom in an intense, high-frequency, pulsed laser field. Under certain conditions, we find that hydrogen has a significant probability of surviving the ramp of the laser field to intensities at which ionization is strongly suppressed. We examine the dynamics of this process and discuss a method to observe it experimentally.

We compare calculated optical harmonic spectra for the hydrogen molecule and the hydrogen atom in an intense, linearly polarized laser field. The hydrogen atom calculations use an exact, time-dependent method and the hydrogen molecule calculations are performed in a time-dependent Hartree-Fock approximation. In both cases, the laser-matter interaction is treated completely non-perturbatively. We show that if the H 2 bond length is stretched so that H 2 has the same ionization potential as H, both systems produce remarkably similar harmonic spectra when irradiated with a 1064 nm laser at 1 × 10 14 or 2 × 10 13 W/cm 2 .

Cadmium vapour contained together with a noble gas in a quartz fluorescence cell at a controlled temperature, was irradiated with pulsed dye-laser radiation which excited the Cd atoms to the 53P1 state. The effective decay rate of the 53P1 atoms was determined in relation to the Cd or noble-gas densities by methods of fluorescence spectroscopy with time resolution. The experiments yielded the total depopulation cross sections of 167, 1.1 × 10−3, 3 × 10−4, 6.1 × 10−4, 1.3 × 10−3 and 3.3 × 10−3 A2 for collisions with ground-state Cd, He, Ne, Ar, Kr and Xe atoms, respectively.

Zinc vapour pure or mixed with a noble gas in a quartz cell at a controlled temperature, was irradiated with pulses of dye-laser radiation which excited the Zn atoms to the 4 3 P 1 state. The effective decay rate of the 4 3 P 1 atoms was determined in relation to Zn or noble-gas densities by methods of fluorescence spectroscopy with time resolution. The experiments yielded the radiative lifetime of the 4 3 P 1 state (τ 0 = 27 μs), and the total 4 3 P 1 depopulation cross sections of 205, 0.2, 2.6 × 10 −3 , and 5.3 × 10 −3 A 2 for collisions with ground-state Zn, He, Ar and Xe atoms, respectively. The measurement of τ 0 was verified in a separate experiment employing a supersonic expansion beam of Zn atoms seeded in Ar

We employ a method for solving the time-dependent Schrodinger equation for an atom or molecule in an intense, pulsed-laser field to study multiphoton emission processes. Single-electron effective potentials have been developed that reduce the computational effort required for multielectron systems. Illustrative results for electron and photon emission rates from krypton at 532 nm in the intensity range (1‒4) × 1013 W/cm2 are presented and compared to recent measurements.

The 0 + u (4 1 P) state of Zn 2 was excited near the crossing of the 1 Σ + u (4 1 P) and 3 Π u (4 3 P) states, using pump and probe methods with time resolution. A structured excitation spectrum was observed in the 530–570 nm region and is ascribed to the 3 Π u (4 3 P)← 3 Π g (4 3 P) vibronic transitions. A fluorescence spectrum which was emitted in the 250–306 nm region is believed to arise from the 1 Σ + u →X 1 Σ + g bound—free decay. It consists of a “Condon internal diffraction pattern” whose profile suggests that it was emitted from a high vibrational level ( v ′ = 19) of the 1 Σ + u state.

Laser catalysis of the collinear H + H2 exchange reaction is computed with an exact quantum mechanical method. The radiative process is treated with a partitioning theory developed previously, and the scattering calculations are performed using a coupled channels expansion in the artificial channel method. By studying the optical line shape as a function of the scattering energy, the signature of the scattering resonances on the optically induced reaction is unraveled. We show that when the collision energy is tuned to a resonance, laser catalysis results in selective vibrational excitation of the product H2 molecule. Implications of this effect for past and future experiments are discussed.

The relative intensities of the vibrational components in the laser-induced fluorescence spectrum arising from a particular molecular vibronic state are usually independent of the exciting wave-length and thus do not depend on the lower vibronic state from which the excitation takes place. We have observed that although the intensity pattern in the fluorescence spectrum from the E0 − (ν'=0) level is independent of the excitation wavelength, the relative intensities of the bands in the fluorescence spectrum from the E0 − (ν'=1) level do depend on the wavelength of the exciting light. We have also observed that the vibrational intensity distribution in the excitation spectrum of the E0 − (ν'=1) level depends on the fluorescence wavelength at which it is monitored, whereas the intensity profile in the excitation spectrum of the E0 − (ν'=0) level is independent of the monitoring wavelength

Excitation and fluorescence spectra arising from bound-bound transitions between the E${0}^{\mathrm{\ensuremath{-}}}$ and the A1, A${0}^{\mathrm{\ensuremath{-}}}$, and B${0}^{\mathrm{\ensuremath{-}}}$ states of the HgZn excimer were investigated using pump-and-probe methods of laser spectroscopy. Several previously unknown bands were recorded, interpreted, and analyzed, yielding vibrational constants, relative energies, and relative internuclear separations for the various states.

Radiative transitions between the A2Π and X2Σ+ states of the ZnH molecule were induced using pump-and-probe methods of laser spectroscopy and resulted in the observation of several fluorescence and excitation bands with rotational structure. A time-evolution study of the fluorescence yielded the radiative lifetime τ0 = 47 ± 3 ns for the A2Π state; the cross section for quenching the A2Π, ν = 0 state by Zn collisions was estimated to be (2.4 ± 0.3) × 10−15 cm2. We also concluded that the ground-state ZnH molecule is a transient species that exists for approximately 10 μs.

We report observations of inelastic collisions between low-lying sodium states and argon in its ground state. The sodium atoms were pumped to the 5S or 4D state by two-photon absorption of broadband radiation from a flashlamp-pumped dye laser with a pulse duration of the order of 1 μsec. Experiments were carried out in a vapour cell with an Na density of 1012cm-3 and with argon densities up to about 2 x 1018 cm-3 (100 torr). Cross sections (both total and state-specific) were determined by measuring the fluorescence intensity from the various states relative to the fluorescence intensity of the laser-excited state and by studying this ratio as a function of the perturber pressure. The following cross sections were obtained: σ 4D−4P = 0.3 ± 0.1 A 2 , σ 5S−4P = 0.10 ± 0.03 A 2 , σ 4P−3D = 2.2 ± 0.7 A 2 , ( σ 4P ) total = 4.1 ± 0.5 A 2 and (σ 3D ) total = 1.0 ± 0.4 A 2 .

We show how the method of coherent control can be applied to control product yield in bimolecular chemical reactions. First, a laser pulse prepares a coherent superposition of two bound levels of an electronically excited state of the reactants. Then, after a variable delay, a second laser pulse photodissociates the system to the ground state continuum. By varying the detuning of the excitation pulse and the delay between the two pulses, we show that significant control of the product ratio can be achieved. As a first application of the method we demonstrate, with exact quantum‐mechanical calculations, the control of the collinear H+H2 and D+H2 reactions. The pulses and delays used in these calculations are on the order of picoseconds, a range of time scales well within that achievable with conventional laser systems.

We present a theoretical investigation of energy transfer in the phenylene ethynelene dendrimer known as the nanostar. Data from extensive molecular dynamics simulations are used to model the dynamical effects caused by torsional motion of the phenyl groups. We compare rate constants for energy transfer between the two-ring chromophore and the three-ring chromophore obtained via the Forster model, the ideal dipole approximation (IDA), and the transition density cube (TDC) method, which has as its limit an exact representation of the Coulombic coupling. We find that the rate constants obtained with the TDC are extremely sensitive to the phenyl group rotation, whereas the constants computed with the Forster model and the IDA are not. The implications of these results for the interpretation of recent pump-probe experiments on the nanostar are discussed in detail. Finally, we predict the temperature dependence of the rate constant for energy transfer.

We study the ionization of extreme Stark states in sodium by THz electromagnetic half-cycle pulses. The results of our full-quantum calculations reveal the presence of an oscillatory pattern in the ionization spectra of extreme red (downhill) states that have been kicked away from the ion core (downhill with respect to the potential imposed by the Stark field). We find no oscillations in the spectra of extreme blue (uphill) states that have been kicked towards the ion core (also downhill with respect to the Stark potential). The oscillatory pattern in the red state ionization spectra is explained with a one-dimensional semiclassical model in terms of interferences between two classical paths. This model also predicts that the blue state ionization spectra should also show oscillatory behavior. The absence of the oscillations in the full calculations can be regarded as a breakdown of the one-dimensional model in representing the ionization dynamics of these states. We find that the one-dimensional model fails when the duration of the THz pulse is comparable to the classical Kepler orbit time.