Single-photon Excitation Research Articles

A vacuum ultraviolet laser single-photon zero kinetic energy photoelectron spectroscopic study of the 2E3/2 ground electronic state of CH3Br+

The vacuum ultraviolet laser single-photon zero kinetic energy (ZEKE) photoelectron spectrum of the [Formula: see text]2E3/2 ground electronic state of the methyl bromide cation is reported. The spectrum is dominated by the origin band 000 of the transition [Formula: see text]2E3/2 ← [Formula: see text]1A1. In addition, the 210 band and the 311 hot band are observed. All observed bands show similar rotational contours. Simulation of the rotational contour of the origin band yields the first ionization energy of methyl bromide (85 031.2 ± 1.0 cm–1) and the rotational constants of the cation in its ground electronic state. Key words: methyl bromide, vacuum ultraviolet laser, single-photon excitation, zero kinetic energy photoelectron spectroscopy.

The Jahn-Teller effect in the lower electronic states of benzene cation. III. The ground-state vibrations of C6H6+ and C6D6+.

New mass analyzed threshold ionization (MATI) spectra of the molecules C(6)H(6) (+) and C(6)D(6) (+) have been collected using tunable vacuum ultraviolet (VUV) single photon excitation from the neutral ground state and also using two-photon excitation through the 6(1) vibration of the (1)B(2u) S(1) state. Emphasis was placed on obtaining accurate relative intensities of the vibrational lines in order to use this information in the vibronic analysis. The MATI spectra collected from VUV (S(0) originating state), triplet (T(1)), and resonant two photon (S(1)) excitation schemes were compared with Jahn-Teller calculations employing the classical model of Longuet-Higgins and Moffitt to obtain the Jahn-Teller coupling parameters of 3 of the 4 linearly active modes (e(2g) modes 6-9 in Wilson's notation). Franck-Condon factors, including the effects of geometry changes, were calculated from the vibronic wave functions and used to identify the lines in the various spectra. It is found that most of the lines with substantial intensity can be understood using only the modes 1, 6, 8, and 9. Weaker peaks are due to various non-e(2g) modes, but these do not derive intensity through Jahn-Teller coupling. When the effects of geometry change were included, simulations of the spectra from the calculated vibrational energies and intensities were close to the experimental spectra. This verifies the applicability of the model to the understanding of the vibrational structure of this type of molecule, but some variations indicate directions for further improvement of the model.

Two-photon lasing of dye-doped photonic crystal lasers

We report two-photon pump lasing action of a photonic crystal laser, which comprises fluorescent dye-doped chiral nematic liquid crystals. The dye absorbs light peaking at 466 nm, while the pumping was conducted at 1000 nm wavelength. Due to redistribution of density of states by a stopping band, a strong single-mode stimulated emission was observed at the band edge wavelength of 608 nm with 1 nm detection-limited linewidth. A comparative study shows that under an identical pumping geometry, the two-photon lasing threshold is only 120 times larger than that for the single-photon excitation, a ratio lower than those generally reported for up-conversion lasing.

Complex rotation method for the Dirac Hamiltonian

We show that the method of the complex rotation can be applied to the Dirac Hamiltonian. We compute positions and widths of resonance levels of hydrogenlike ions ~with Z51 and Z510), described relativisti- cally, in the presence of the uniform dc electric field ~relativistic Stark effect !. We show that, for small Z, positions of the resonance levels, given by the relativistic approach, agree well with known nonrelativistic results. The relativistic treatment introduces corrections on the same order both for the resonance positions and widths, and may, therefore, alter widths considerably. The complex rotation method is a well-known tool, allow- ing to calculate positions and widths of resonances in atomic systems @1-7#. To mention just a few applications, the method has been used in nonrelativistic quantum-mechanical calculations of resonance parameters of doubly excited states @8,9# or Stark resonances @10-12# in such systems as He, H 2 ,o r Ps 2 . The above mentioned applications of the complex rotation procedure neglected relativistic effects. Recent developments of the experimental technique perfected experimental data to a degree where relativistic effects become observable. For example, in recent publications @13,14# of 2lnl8 resonances in helium the authors show that theoretical predictions based on the nonrelativistic approach differ considerably from the experimental data. Only by inclusion of the spin-orbit effects in the scheme of an R-matrix multichannel quantum-defect calculation @15# were the experimental results explained. Another manifestation of the relativistic effects has been reported in Ref. @16#, where a peak in the electron-He 1 (1s) scattering has been observed, which is forbidden in the pure LS-coupling scheme. The recent experimental studies @17# of the effect of the strong electric field on helium hint at the important role played by the relativistic effects. Also, forbid- den in the LS coupling, doubly excited states have been ob- served in a single-photon excitation from the ground state of helium @18#. These findings indicate that, in the spectra of the doubly excited states, or in the process of the interaction of atomic states with electric field, relativistic effects may play quite an important role. Incorporation of the relativistic effects into the scheme of calculation and, in particular, proper modifi- cations of the complex rotation procedure, allowing to take full account of the relativistic effects, seem to be necessary. In the present paper we propose a modification of the complex rotation method, taking full account of the relativ- istic effects. We develop the complex rotation procedure for the Dirac Hamiltonian.

Three-dimensional differentiation of photo-autotrophic biofilm constituents by multi-channel laser scanning microscopy (single-photon and two-photon excitation)

A simple microscopic method to three-dimensionally differentiate between various members in photo-autotrophic biofilm systems is described. By dual-channel single-photon (confocal) and two-photon laser scanning microscopy, the signals in the red and far red channels as well as their combination can be simultaneously recorded. The method takes advantage of the autofluorescent signal of cyanobacteria-recorded in the red and far red channel and the autofluorescent signal of the green algae-recorded in the far red channel only. The differentiation is based on the specific pigment composition of cyanobacteria and green algae in combination with the appropriate filter settings for detection of the autofluorescent emission signals. The method allows the non-destructive, three-dimensional examination of fully hydrated interfacial microbial communities at high resolution as well as the clear separation between autofluorescent signals of cyanobacteria and green algae. Furthermore, there is a third option to record additional signals simultaneously such as nucleic acid stained bacteria, bacteria labeled with phylogenetic probes or glycoconjugates stained by using lectins. With state of the art laser scanning microscopes, even a fourth channel is available for recording yet another parameter, e.g. in the reflection (single-photon only) or fluorescence (single- and two-photon) mode. Thus the approach represents a convenient tool to study multiple parameters of complex photo-autotrophic biofilm systems.

Long-range molecular resonances in a cold Rydberg gas.

We present evidence for molecular resonances in a cold dense gas of rubidium Rydberg atoms. Single UV photon excitation from the 5s ground state to np Rydberg states (n=50-90) reveals resonances at energies corresponding to excited atom pairs (n-1)d+ns. We attribute these normally forbidden transitions to avoided crossings between the long-range molecular potentials of two Rydberg atoms. These strong van der Waals interactions result in avoided crossings at extremely long range, e.g., approximately 58 000 times the Bohr radius (a(0)) for n=70.

Multichannel quantum defect theory and high-resolution spectroscopy of the hyperfine structure of high Rydberg states of83Kr

The hyperfine structure of ns and nd Rydberg states of {sup 83}Kr has been measured in the range n=30-190 below the {sup 2}P{sub 3/2} ionization threshold by pulsed-field ionization following single-photon excitation from the {sup 1}S{sub 0} ground state using a narrow-bandwidth vacuum-ultraviolet laser system. A multichannel quantum defect theory (MQDT) treatment of the hyperfine structure in Rydberg states of the rare-gas atoms has been developed that quantitatively accounts for the effects of the nuclear spin on the spectral structures over the entire range of principal quantum number investigated. The model allows the parametrization of the hyperfine structure of the Rydberg states in terms of the ionic hyperfine structure and relies on the assumption that the interaction with the nuclear spin is negligible in the close-coupling region of the electron-ion collision, an assumption that is also expected to be valid in other atomic and molecular systems. Improved eigen quantum defects for the ns and nd Rydberg series with J=1 and 2 have been derived from the MQDT analysis, and the hyperfine structure of the two {sup 2}P{sub 3/2} and {sup 2}P{sub 1/2} spin-orbit components of the ground state of {sup 83}Kr{sup +} has been determined.

Transition of Femtosecond Laser Ablation Mechanism for Sodalime Glass Caused by Photoinduced Defects

Femtosecond laser ablation mechanism was systematically investigated on sodalime glass in ambient conditions. The ablation crater diameter was measured for varying numbers of laser pulses as for varying well as the laser fluence. The analysis of the results with a one dimensional spatial Gaussian fluence distribution reveals that the inherent ablation mechanism has been altered from a multi-photon process to a single photon excitation due to defect sites that have been accumulated by successive laser pulses. Furthermore, the transition between the two regimes was found to be a function of both the laser fluence and the number of laser shots.

Single molecule photodynamics by means of one- and two-photon approach

Single molecule spectroscopy allows to investigate heterogeneous behaviours on photochemical and structural grounds. We report on studies of the effect of the excitation intensity on the internal photodynamics of simple dyes immobilized on chemically etched glass slides. The use of the excitation intensity needed for two-photon excitation induces local heating, structural changes and transitions to dark states. Similar behaviour is found on single green fluorescent proteins immobilized on glass slides or embedded in silica gels upon single-photon excitation. However, by sampling the images with sufficiently low frequency, we are able to follow relevant biological events, such as the unfolding kinetics. We find that the glass slides are preferable in terms of the signal-to-noise ratio but the protein is not preserved in its native state, while evidence for the native conformation of the single proteins in the silica gels is found in the uniformity of the fluorescence emission.

Primary spherical aberration in two-color (two-photon) excitation fluorescence microscopy with two confocal excitation beams.

We study the effects of primary spherical aberration on the three-dimensional point spread function (PSF) of the two-color (two-photon) excitation (2CE) (2PE) fluorescence microscope with two confocal excitation beams that are separated by an angle theta. The two excitation wavelengths lambda1 and lambda2 are related to the single-photon excitation wavelength lambda(e) by: 1/lambda(e) = 1/lambda1 + 1/lambda2. The general case is considered where both focused beams independently suffer from spherical aberration. For theta = 0, pi/2, and pi, the resulting deterioration of the PSF structure is evaluated for different values of the spherical aberration coefficients via the Linfoot's criteria of fidelity, structural content, and correlation quality. The corresponding degradation of the peak 2CE fluorescence intensity is also determined. Our findings are compared with that of the 2PE fluorescence (lambda1 = lambda2) under the same aberration conditions. We found that the 2CE microscope is more robust against spherical aberration than its 2PE counterpart, with the pi/2 configuration providing the clearest advantage. The prospect of aberration correction in the two-beam 2CE microscope is also discussed.

High-resolution threshold-ionization spectroscopy of NH3

High-resolution photoionization, zero-kinetic-energy photoelectron and Rydberg-state-resolved threshold-ionization spectra of ammonia and its deuterated isotopomers have been recorded in the region of the lowest vibrational levels (v2+=0,1) of the X̃+ ground ionic state of NH3+ following single-photon excitation from the ground neutral state using a narrow bandwidth vacuum ultraviolet laser system (bandwidth 0.008 cm−1). The resolution enables the observation of photoionization transitions originating from distinct tunneling components of the ground neutral state and the measurement of the spin-rotational splittings of the ionic energy levels. A new value of the first adiabatic ionization potential of NH3 [I.P.=82 158.751(16) cm−1] has been derived which is more accurate than previous values by almost two orders of magnitude. The photoionization dynamics of NH3 to the lowest vibrational levels of the X̃+(2A2″) ground state of NH3+ is dominated by the emission of even ℓ photoelectron partial waves, and a strong propensity rule is observed that favors transitions with ΔK=K+−K″=0 and, to a lesser extent, ΔK=3.

Fluorescence resonance energy transfer in a novel two-photon absorbing system.

A novel fluorescence resonance energy transfer (FRET) system containing a two-photon absorbing dye and a nile red chromophore has been synthesized. Upon two-photon excitation by laser at 815 nm this molecule displays efficient energy transfer from the two-photon absorbing dye to the nile red moiety, with an 8-fold increase in emission compared to the model compound. Similarly, single-photon excitation of the two-photon absorbing moiety at 405 nm results in >99% energy-transfer efficiency, along with a 3.4-fold increase in nile red emission compared to direct excitation of the nile red chromophore at 540 nm. This system provides an effective way to use IR radiation to excite molecules that, by themselves, have little or no two-photon absorption.

Two-photon photothermal deflection spectroscopy of atomic sodium (abstract)

Two-photon photothermal spectroscopy (with nonresonant intermediate state) is a very useful technique, because many atomic and molecular species that are usually optically inaccessible by single-photon excitation can be made accessible by two-photon excitation. Some examples of atomic species are H and O. As a first step, we are investigating photothermal signals generated in sodium vapor produced by two-photon excitation of the 52S1/2 state at 602.23 nm. Sodium vapor is contained in a glass absorption cell and N2 buffer gas at 0.66 amagat pressure is used to collisionally transfer the optical energy from Na to the rotational-translation (thermal) energy of N2 gas. A pulsed dye laser operating at 602.23 nm provides the pump beam. The absorption cell is heated in an oven to obtain about 5×10−4 Torr of Na vapor pressure. Photothermal deflection of a He–Ne laser probe beam is observed. Results of these experiments will be discussed.

Subbandgap laser-induced single event effects: carrier generation via two-photon absorption

Carrier generation based on subbandgap two-photon absorption is demonstrated and shown to be a viable alternative to the conventional single-photon excitation approach in laser-induced single event effects. The two-photon approach exhibits characteristics distinct from those of single-photon excitation, and may be advantageous for a range of single-event effect investigations. The charge track produced by two-photon absorption more closely resembles that of heavy-ion irradiation and, because the photon energy is subbandgap, backside injection through bulk silicon wafers is straightforward and three-dimensional mapping is possible.

Dynamic confocal imaging in acute brain slices and organotypic slice cultures using a spectral confocal microscope with single photon excitation.

Confocal imaging in living brain slices allows the resolution of submicrometre structures of nerve cells, glia and brain vessels. Imaging living brain slices is in many respects different from conventional fixed histological preparations for which confocal microscopes were designed originally. Several problems (i.e. mechanical and thermal drift, and autofluorescence) resulting from the optical and structural properties of brain slices are discussed. Fluorescent indicators may be used to monitor numerous intracellular parameters such as pH and Ca(2+) concentration, but not all of them are equally suitable for this type of work. Genetically engineered fluorescent proteins can be used to visualise the fine dendritic structure of neurones or track particular intracellular structures and proteins. They have also been used to generate indicators for Ca(2+), cAMP and other molecules. While conventional chemical indicators can be either loaded into neurones via patch pipettes or as membrane-permeable esters, protein indicators can be expressed in various types of cells using adenoviral vectors. Adenoviral transgenesis can be performed in vitro in both acute slices and organotypic slice cultures. Organotypic slice cultures give excellent optical access to neurones loaded with either conventional fluorescent indicators or transfected with adenovirus to express fluorescent proteins. They are most suitable for experiments where both conventional and genetically engineered indicators are combined. Single photon imaging in brain slices is limited to the superficial layers (approximately<or=50 microm), while multiphoton excitation has a much greater depth of penetration. However, the overall optical resolution achievable in single photon mode is at least as good as when using multiphoton excitation.

Confocality condition in two-color excitation microscopy with two focused excitation beams

We evaluate the point spread function (PSF) of the two-color excitation (2CE) microscope when the confocality condition is not strictly satisfied by the two focused excitation beams which are separated by an angle θ≠0. The 2CE wavelengths λ 1 and λ 2 are related to the single-photon excitation wavelength λ e of the sample according to: 1/ λ e=1/ λ 1+1/ λ 2. Spatio-temporal simultaneity for the two excitation beams is essential in 2CE ( λ 1≠ λ 2). Only aberration-free misalignment of the geometrical foci is considered and the degradation of the peak 2CE fluorescence intensity ( F 2c p) is determined as a function of the focus separation for θ=0,π/2 and π. The deterioration of the fluorescence PSF is also evaluated using the Linfoot's criteria of fidelity, structural content and correlation quality. We compare the 2CE PSF behavior with that of the two-photon excitation (2PE) fluorescence ( λ 1= λ 2) under the same misalignment settings. At θ=π/2, the F 2c p degrades less rapidly than the peak 2PE fluorescence intensity ( F 2p p) for misalignments that are confined within the focal plane of the focusing lens L1 for λ 1. At θ=π, F 2c p and F 2p p degrade in a similar manner. For misalignments along the optical axis of L1 and at θ=π/2, the F 2c p degrades monotonically while the F 2p p degradation is accompanied by fluctuations caused by two-beam interference. Our calculations reveal that 2CE imaging is more resilient to the ill-effects of misalignment than the two-beam 2PE.

Suppression of angular-momentum mixing in precision solutions of the Schrödinger equation for photoexcitation of Rydberg states

Numerical integration of the Schr\"odinger equation for the resonant single-photon excitation from the $1s$ to $10p$ state in atomic hydrogen has been used to produce solutions converged to a precision of a few parts per million. At all intensities investigated (in the range $10--100{\mathrm{T}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}),$ the population in the states having angular momentum higher than $\mathcal{l}=1$ after a narrowband (500 fs) laser pulse are found to be neglible. This shows that off-resonant states can effectively suppress Raman-like transitions to degenerate states of higher angular momentum. Instead of such mixing, strong Rabi-like oscillations between $1s$ and the $\mathrm{np}$ Rydberg series are observed.

Trapped Brownian Motion in Single- and Two-Photon Excitation Fluorescence Correlation Experiments

We investigate possible trapping effects induced by the infrared radiation employed for two-photon fluorescence spectroscopy and microscopy on the diffusion of small fluorophores and draw a comparison to single-photon excitation. A first-order treatment of the effect of an optical trapping potential on the diffusion time and the number of molecules per excitation volume is also derived. By analyzing the fluorescence fluctuations arising from solutions of small dyes versus the excitation power and comparing them to the results with microspheres, we show that the bias on the molecular diffusion of small dyes is negligible when two-photon excitation is employed. In single-photon excitation, close to the resonance absorption of the dye, a small but detectable bias effect on the diffusion is found, in agreement with the study by Osborne et al. [Osborne, M. A.; Balasubramanian, S.; Furey, W. S.; Klenerman, D. J. Phys. Chem. B 1998, 102 (17), 3160].

High-resolution spectroscopy of HCl and DCl isolated in solid parahydrogen: Direct, induced, and cooperative infrared transitions in a molecular quantum solid

The infrared spectroscopy and rovibrational dynamics of HCl and DCl dopants in solid parahydrogen (pH2) were investigated using high-resolution spectroscopic methods. The absorption spectra of HCl and DCl monomers in solid pH2 closely resemble the corresponding low temperature gas phase spectra, indicating that the gas phase vibrational and rotational quantum numbers of the dopant are conserved within the pH2 solid. Small deviations from gas phase behavior are observed, however, such as a reduced HCl rotational energy level spacing and splitting of the fivefold orientational degeneracy of HCl rotational states with J=2. In addition, the pure vibrational Q1(0) (v=1←0, J=0←0) H2 transition, which is infrared inactive in pure solid pH2, is detected in the HCl doped sample. We propose that this transition is induced in pH2 molecules by neighboring HCl molecules through a weak “overlap induction” mechanism that is the only induction mechanism operative for J=0 impurities in pH2. Rovibrational transitions are also detected near the induced Q1(0) H2 absorption; these are attributed to cooperative transitions involving single photon excitation of pH2–HCl pairs. Detailed isotopic analysis reveals that these cooperative transitions involve pure vibrational excitation of the pH2 and pure rotational excitation of the HCl. Two-molecule transitions have long been studied for isotopic and rotational dopants (e.g., D2, HD, orthohydrogen) in solid pH2, but this is the first time such cooperative transitions have been attributed to a chemical impurity in pH2 matrices.

Photobleaching under single photon and multi-photon excitation: chloroplasts in protoplasts from Arabidopsis thaliana

Photobleaching as a result of intense laser irradiation has been an important issue in the applications of confocal microscopy. In this study we are employing time-lapsed micro-spectroscopy to compare the effects of photobleaching under single photon and multi-photon excitation. A model is derived to account for photobleaching in both cases and protoplasts from Arabidopsis thaliana are employed as the sample. We have found that multi-photon excitation have impacts on the samples in ways much different from cw illumination in the linear domain.