Generation of harmonic radiation by helium atoms in intense laser pulses

Measurements of natural radiative lifetimes in atomic tellurium in the 5p ³7p configuration are reported. Two-step laser excitation was applied on tellurium vapour in a cell that was differentially heated and pumped through a capillary. The decay of the fluorescence signal was recorded at different vapour pressures. The following four radiative lifetime values were obtained: 7p ⁵ P ₁: 128(11) ns, 7p ⁵ P ₂: 99(8) ns, 7p ³ P ₁: 82(10) ns and 7p ³ P ₂: 124(8) ns.     

Multiphoton ionization of magnesium and calcium atoms by short and intense laser pulses

Single and double ionization of magnesium and calcium atoms following Nd: YAG laser multiphoton excitation at 1064 and 532 nm have been studied by employing pulses of 35 ps and 200 ps duration at intensities of the order of 10¹⁰–2×10¹³ W/cm². The dependence of ion formation on the laser intensity has been measured and the slopes of the linear parts of the log-log plots and the ratios of saturation intensities for two pulse durations have been compared with the predictions of the scaling law. No evidence for a pure direct double ionization process has been obtained.     

Unveiling the nonadiabatic rotational excitation process in a symmetric-top molecule induced by two intense laser pulses

We experimentally investigate the nonadiabatic rotational excitation process of a symmetric-top molecule, benzene, in the electronic ground state irradiated by intense nonresonant ultrafast laser fields. The initial rotational-state distribution was restricted mostly to the five lowest levels with different nuclear spin modifications by an extensive adiabatic cooling with the rotational temperature well below 1 K, and distributions after the interaction with a femtosecond double-pulse pair (3-5 TW/cm(2) each with 160 fs duration) with time delays were probed in a quantum-state resolved manner by employing resonant enhanced multiphoton ionization via the S(1) ← S(0) 6(0) (1) vibronic transition. Populations of 10 rotational levels with J ranging from 0 to 4 and K from 0 to 3 were examined to show an oscillatory dependence on the time delay between the two pulses. Fourier analysis of the beat signals provides the coupling strengths between the constituent levels of the rotational wave packets created by the nonadiabatic excitation. These data are in good agreement with the results from quantum mechanical calculations, evidencing stepwise excitation pathways in the wave packet creation with ΔJ = 2 in the K = 0 stack while ΔJ = 1 and 2 in the K > 0 stacks.     

On the limitations of adiabatic population transfer between molecular electronic states induced by intense femtosecond laser pulses

The possibility to perform a stimulated Raman adiabatic passage process in molecules on the ultrafast time scale is investigated theoretically. Motivated by recent experiments, the mid R:B<--mid R:X electronic transitions in molecular iodine are studied as a prototype example with the goal to selectively induce a population transfer employing two intense and time-delayed ultrashort laser pulses and different coupling schemes. For the purpose of interpretation, the coupled multilevel vibronic problem is reduced to a quasi-three-level system by averaging over the vibrational degree of freedom. It is shown that the vibrational dynamics becomes essential at high field intensities. Considering a 2-dimensional parameter space (intensity and delay time of the femtosecond laser pulses), a strong-field control landscape is constructed.     

Quantum efficiency improvement of a cesium based resonance fluorescence detector by helium-induced collisional excitation energy transfer

Quantum efficiency improvement of a cesium based resonance fluorescence detector (RFD) was achieved by enhancing the transfer in a particular channel of the RFD excitation scheme with noble gas-induced collisional excitation energy transfer (CEET). The influence of Cs–Ar and Cs–He collisional mixing between the 6D and 7P states in cesium on the quantum efficiency of the 6S → 6D → 7P → 6S excitation scheme was investigated by fluorescence measurements at relevant transitions. Ar-induced CEET was found to have little effect on the fluorescence response and quantum efficiency of the Cs RFD excitation scheme. However, a 35 fold quantum efficiency increase in the cesium resonance fluorescence detector response at only moderate He pressures was observed.     

The response of a flame ionisation detector to pulses of noble gases in a gas chromatograph

Summary It has been confirmed that pulses of noble gases injected into the nitrogen carrier gas in a gas chromatograph produce responses from a flame ionisation detector. The responses are caused by a dual perturbation of the ‘blank’ signal resulting from trace impurities in the nitrogen. One type of perturbation is explicable from the theory of vacancy chromatography whilst the other, it is suggested, results from the influence of the noble gas on the net rate of production of charge carriers in the flame. This latter response can be used for gas hold up time measurements.     

Classical path study of excitation of a collision system by ultrashort laser pulses

The interaction of an ultrashort laser pulse with a two-level collision system is investigated. An increased overall photon efficiency of ultrashort pulses is confirmed in many cases by the calculation. The dependence of the excitation process on duration, intensity and shape of the laser pulse is studied on the basis of calculations with the classical-path method. Realistic potentials modeling theXΣ → AΠ transition in Na-Ar are employed. Numerical trajectories were generated on these potentials that either switch at the Condon points from the ground state to the excited state or are propagated on the average potential. The role of the nonresonant excitation of free atoms is discussed in detail and found to be an important factor accompanying collision pair excitation. However a drastic reduction of this effect occurs when considering the propagation of the pulse through the medium before collision. Parallel computation of related collisions leads to a calculational procedure by which the necessary average over the pulse onset time is performed efficiently.     

Mediation of resonance energy transfer by a third molecule

The influence of a third molecule on the rate of resonance energy transfer is studied using diagrammatic perturbation theory within the framework of molecular quantum electrodynamics. Two distinct mechanisms are identified. One corresponds to direct transfer between donor and acceptor while the other involves relay of energy by the third species. Fermi Golden rule transition rates valid for all separation distances beyond wave function overlap are evaluated for these two processes as well as for the interference term between direct and indirect exchange, thereby extending previous work which was limited to the near-zone only. Short- and long-range limits are also obtained in each case. It is found that in the near-zone the indirect rate contribution exhibits inverse sixth power dependence on relative distances of emitter and absorber relative to the third body, in contrast to its far-zone counterpart, which exhibits inverse square behavior. The interference term, however, displays inverse cubic dependence on all three distance vectors at short-range and inverse behavior in the far-zone. Interestingly, for a collinear arrangement of the three molecules in the near-zone, the interference term is negative, reducing the overall rate of energy transfer. The results obtained are interpreted in terms of microscopic and macroscopic pictures of transfer occurring within a surrounding medium.     

Semiclassical IVR approach to rotational excitation of non-polar diatomic molecules by non-resonant laser pulses

We apply the ‘classic’ semiclassical initial value representation (SC-IVR) approach to describe rotational excitation of non-polar diatomic molecules by intense short non-resonant laser pulses. We also investigate the applicability of the quantum mechanical sudden approximation. It is found that the SC-IVR approach gives accurate rotational excitation probabilities in regimes where the sudden approximation fails. Primitive semiclassical wavefunction propagation is used to illustrate the phenomenon of angular focussing of rotation states by strong pulses.     

Characterisation of antibodies and analytes by surface plasmon resonance for the optimisation of a competitive immunoassay based on energy transfer

The determination of binding constants using surface plasmon resonance (SPR) was introduced to optimise a competitive homogeneous fluorescence energy-transfer immunoassay (ETIA) before labelling. Steroids were chosen as model for the detection of three analytes estrone, estradiol and ethinylestradiol--by taking three polyclonal antibodies (anti estrone-, anti estradiol- and anti estrogen-antibodies) and the corresponding analyte derivatives used for the immunisation. The active concentration of the antibodies was determined before and after labelling. Inhibition curves were recorded using SPR for all possible combinations of analyte, antibody, and analyte derivatives. The experiments revealed that the active antibody concentration can be reduced to 30% whereas the antibody affinity is not affected by the labelling process. Limits of the use of SPR for determination of affinity constants in solution are discussed. All possible ETIA calibration for the quantification of estrone and estradiol was performed. The lower limits of detection for estrone (0.06 microg L(-1)) and estradiol (0.17 microg L(-1)) were reached with the anti-estrogen IgG and its derivative     

A resonance mechanism of efficient energy transfer mediated by Fenna-Matthews-Olson complex

The Wigner-Weisskopf-type model developed by Alicki and Giraldi [J. Phys. B 44, 154020 (2011)] is applied to the biological process of energy transfer from a large peripheral light harvesting antenna to the reaction center. This process is mediated by the Fenna-Matthews-Olson (FMO) photosynthetic complex with a remarkably high efficiency. The proposed model provides a simple resonance mechanism of this phenomenon employing exciton coherent motion and is described by analytical formulas. A coupling to the vibrational environment is a necessary component of this mechanism as well as a fine-tuning of the FMO complex Hamiltonian. The role of the relatively strong coupling to the energy sink in achieving the resonance condition and the absence of heating of the vibrational environment are emphasized.     

Absolute detection of metastable rare gas atoms by a cw laser photoionization method

A novel, accurate method for the absolute detection of metastable rare gas atoms is described and demonstrated. It involves a direct in situ determination of the electron emission coefficient γ for impact of the respective metastable atom on a conducting surface. γ is reliably obtained by a cw two-photon ionization — depletion technique: the reduction ΔI _S in electron current from the detector surface due to efficient photoionization removal of the metastable flux is compared with the photoelectron current ΔI _P (γ = ΔI _S/ΔI _P). The principle of the method, possible realization schemes for the different metastable rare gas atoms and the apparatus are described in detail. The method has been applied so far to metastable Ne* (3s ³ P ₂), Ar* (4s ³ P ₂), and Kr* (5s ³ P ₂) atoms, and corresponding results for γ, obtained with five different chemically clean, polycrystalline surface materials and at two surface temperatures (300 K, 360 K) are reported. Whereas for Ne*, the value of γ (≈0.35) showed only a rather weak dependence on the surface material and temperature (as also found for a mixed He* (2³ S, 2¹ S) beam), strong variations in γ, especially at 300 K, were detected for Ar* and Kr* (values between 0.25 and 0.003). Some applications of the described method, especially with regard to the determination of absolute reaction cross sections involving metastable rare gas atoms, are discussed.     

Electronic energy trapping in a crystal due,to a dopant of higher excitation energy than the host

An excimer emitting crystal (9-cyanoanthracene) doped with a guest molecule (9-methoxyanthracene) having its first singlet level ca. 2000 cm^?1 above the host singlet exciton band exhibits efficient energy trapping as demonstrated by host sensitized, red-shifted emission and hetero-photodimerization. It is considered that the trapping is due to exciplex formation between host and guest molecules.     

Self-assembly dynamics of a cylindrical capsule monitored by fluorescence resonance energy transfer

The constituent cavitands of a cylindrical capsule were labeled with donor and acceptor fluorophores, and fluorescence resonance energy transfer (FRET) was employed as a tool to study the dynamics of self-assembly. When donor and acceptor dyes are present in the same capsular assembly, they are brought within 25 A of each other, a distance suitable for efficient energy transfer to occur between them. This allowed for the study of interacting species at nanomolar concentrations providing information unattainable from NMR experiments. The kinetic stability of the capsule in the presence of various guest molecules was investigated which revealed a range of more than 4 orders of magnitude in the rates of cylindrical capsule exchange. While the thermodynamic stability of the capsule generally dictates the self-assembly dynamics, it was discovered that longer rigid guests can impart a significant kinetic barrier to monomer exchange.     

Resonance energy transfer from a fluorescent dye molecule to plasmon and electron-hole excitations of a metal nanoparticle

The authors study the distance dependence of the rate of electronic excitation energy transfer from a dye molecule to a metal nanoparticle. Using the spherical jellium model, they evaluate the rates corresponding to the excitation of l=1, 2, and 3 modes of the nanoparticle. The calculation takes into account both the electron-hole pair and the plasmon excitations of the nanoparticle. The rate follows conventional R(-6) dependence at large distances while small deviations from this behavior are observed at shorter distances. Within the framework of the jellium model, it is not possible to attribute the experimentally observed d(-4) dependence of the rate to energy transfer to plasmons or electron-hole pair excitations.     

Preferential energy transfer from N2(A) to specific energy levels of Cu atoms

Emission spectra resulting from reaction of “clean” N₂(A³ Σ_u⁺) with copper atoms were studied using a flowing afterglow apparatus. The population distribution of the Cu states was calculated from the spectrum; it indicates that Cu atoms are excited by nearly resonant energy transfer processes. N₂(A,v') + Cu(²S_{$\text{1}\text{2}$}) → N₂(X, v) + Cu^* , and that the transfer is most efficient for N₂(A,v') → N₂(X,v) transitions with large Franck-Condon factors. The preferential energy transfer results in population inversion between some of the Cu states.     

A model calculation on the transport of excitation energy in a molecular crystal

We report a model calculation of the transport of a local (site) excitation in a doped molecular crystal containing one impurity. We do not consider the impurity as a direct trap for electronic excitations (zero trap depth) but assume that exciton-phonon interaction is exclusively given by the coupling of excitons with the vibrational displacement of the impurity. The dynamical problem is solved by using a time-dependent effective potential consisting of equilibrium average exciton-phonon interaction and fluctuations around this average. Two correlation functions are computed using the slow phonon limit and assuming that the temperature of the system is 300 K. Transmission of the excitation energy over a distance of eight spacings takes place, electronically, within a few picoseconds. With the exciton-phonon interaction switched on, calculated correlation functions diminish very rapidly with increasing time, indicating that an irreversible transfer of excitonic energy to the thermal bath takes place. Thus transmission of the excitation energy over such a distance (and without a high rate of trapping) is not an efficient process.     

Generation of intense secondary pulsed molecular beams of high kinetic energy controllable by a powerful IR laser

A method for generation of intense secondary pulsed molecular beams and beams of radicals of high kinetic energy controllable by a powerful IR laser is described. A pressure shock (shock wave) is used as a source of secondary beams. The pressure shock is formed in interaction between an intense pulsed supersonic molecular beam (or flow) and a solid surface. The characteristics of the secondary beams were studied. Their intensities and the degree of gas cooling in them were shown to be comparable with the corresponding characteristics of the unperturbed primary beam. The acceleration of molecules in the secondary beam is achieved due to vibrational excitation of them by high-power IR laser pulse in the pressure shock and subsequent vibrational to translational (V–T) relaxation, which occurs when a gas expands through the orifice into a vacuum. Intense [10²⁰ molecules/(sr s)] beams of SF₆ and CF₃I molecules with kinetic energies approximately equal to 1.5 and 1.2 eV, respectively, were generated in the absence of carrier gases. The SF₆ molecular beams with kinetic energies approximately from 2.5 to 2.7 eV with carrier gases H₂, He and CH₄ (SF₆/carriergas=1/10) were obtained. The possibility of generation of intense beams of cold radicals by this method is demonstrated. The intense beams of cold and accelerated CF₃ radicals were generated when the CF₃I molecules in the shock were dissociated by high-power CO₂ laser radiation. The spectral and energetic characteristics of acceleration of SF₆ and CF₃I molecules in the secondary beams were studied. The optimal conditions were found for obtaining high-energy molecules.     

Selective excitation of coupled CO vibrations on a dissipative Cu(100) surface by shaped infrared laser pulses

In a previous paper [Beyvers et al., J. Chem. Phys. 124, 234706 (2006)], the possibility to mode and state selectively excite various vibrational modes of a CO molecule adsorbed on a dissipative Cu(100) surface by shaped IR pulses was examined. Reduced-dimensionality models with stretching-only coordinates were employed to do so. This model is now extended with the goal to include rotational modes. First, we present an analysis of the bound states of the adsorbed CO molecule in full dimension; i.e., six-dimensional eigenstates are obtained by diagonalizing the six-dimensional Hamiltonian containing the semiempirical potential of Tully et al. [J. Vac. Sci. Technol. A 11, 1914 (1993)]. This is achieved by using a contracted iterative eigensolver based on the coupled two-term Lanczos algorithm with full reorthogonalization. Reduced-dimension subsystem eigenvectors are also computed and then used to study the selective excitation of the molecule in the presence of dissipation within the density matrix formalism for open systems. In the density matrix propagations, up to four degrees of freedom were included, namely, r (the C-O distance), Z (the molecule-surface distance), and phi and theta (the azimuthal and polar angles of the molecular axis with respect to the surface). Short, intense laser pulses are rationally engineered and further refined with optimal control theory, again with the goal for mode and state selective excitation. Also, IR-laser induced desorption is studied. For the calculations, the previous two-mode (r,Z) dipole surface is extended to include the angular dependence and the model for the coupling of the molecule to the surface electronic degrees of freedom is refined.