XUV free-electron laser desorption of NO from graphite (0001)

We report results of femtosecond laser induced desorption of NO from highly oriented pyrolytic graphite using XUV photon energies of hν = 38 eV and hν = 57 eV. Femtosecond pulses with a pulse energy of up to 40 μJ and about 30 fs duration generated at FLASH are applied. The desorbed molecules are detected with rovibrational state selectivity by (1 + 1) REMPI in the A(2)Σ(+) ← X(2)Π γ-bands around λ = 226 nm. A nonlinear desorption yield of neutral NO is observed with an exponent of m = 1.4 ± 0.2. At a fluence of about 4 mJ/cm(2) a desorption cross section of σ(1) = (1.1 ± 0.4) × 10(-17) cm(2) is observed, accompanied with a lower one of σ(2) = (2.6 ± 0.3) × 10(-19) cm(2) observable at higher total fluence. A nonthermal rovibrational population distribution is observed with an average rotational energy of = 38.6 meV (311 cm(-1)), a vibrational energy of = 136 meV (1097 cm(-1)) and an electronic energy of = 3.9 meV (31 cm(-1)).     

Stark-assisted population control of coherent CS(2) 4f and 5p Rydberg wave packets studied by femtosecond time-resolved photoelectron spectroscopy

A two-color (3+1(')) pump-probe scheme is employed to investigate Rydberg wave packet dynamics in carbon disulfide (CS(2) (*)). The state superpositions are created within the 4f and 5p Rydberg manifolds by three photons of the 400 nm pump pulse, and their temporal evolution is monitored with femtosecond time-resolved photoelectron spectroscopy using an 800 nm ionizing probe pulse. The coherent behavior of the non-stationary superpositions are observed through wavepacket revivals upon ionization to either the upper (12) or lower (32) spin-orbit components of CS(2) (+). The results show clearly that the composition of the wavepacket can be efficiently controlled by the power density of the excitation pulse over a range from 500 GWcm(2) to 10 TWcm(2). The results are consistent with the anticipated ac-Stark shift for 400 nm light and demonstrate an effective method for population control in molecular systems. Moreover, it is shown that Rydberg wavepackets can be formed in CS(2) with excitation power densities up to 10 TWcm(2) without significant fragmentation. The exponential 1e population decay (T(1)) of specific excited Rydberg states are recovered by analysis of the coherent part of the signal. The dissociation lifetimes of these states are typically 1.5 ps. However, a region exhibiting a more rapid decay ( approximately 800 fs) is observed for states residing in the energy range of 74 450-74 550 cm(-1), suggestive of an enhanced surface crossing in this region.     

Communication: Time-domain measurement of high-pressure N2 and O2 self-broadened linewidths using hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering

The direct measurement of self-broadened linewidths using the time decay of pure-rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps RCARS) signals is demonstrated in gas-phase N(2) and O(2) from 1-20 atm. Using fs pump and Stokes pulses and a spectrally narrowed ps probe pulse, collisional dephasing rates with time constants as short as 2.5 ps are captured with high accuracy for individual rotational transitions. S-branch linewidths of N(2) and O(2) from ~0.06 to 2.2 cm(-1) and the line separation of O(2) triplet states are obtained from the measured dephasing rates and compared with high-resolution, frequency-domain measurements and S-branch approximations using the modified exponential gap model. The accuracy of the current measurements suggests that the fs/ps RCARS approach is well suited for tracking the collisional dynamics of gas-phase mixtures over a wide range of pressures.     

Coherent control of the isomerization of retinal in bacteriorhodopsin in the high intensity regime

Coherent control protocols provide a direct experimental determination of the relative importance of quantum interference or phase relationships of coupled states along a selected pathway. These effects are most readily observed in the high intensity regime where the field amplitude is sufficient to overcome decoherence effects. The coherent response of retinal photoisomerization in bacteriorhodopsin to the phase of the photoexcitation pulses was examined at fluences of 10(15) - 2.5 ×?10(16) photons per square centimeter, comparable to or higher than the saturation excitation level of the S(0) - S(1) retinal electronic transition. At moderate excitation levels of ～6 ×?10(15) photons/cm(2) (<100 GW/cm(2)), chirping the excitation pulses increases the all-trans to 13-cis isomerization yield by up to 16% relative to transform limited pulses. The reported results extend previous weak-field studies [Prokhorenko et al., Science 313, 1257 (2006)] and further illustrate that quantum coherence effects persist along the reaction coordinate in strong fields even for systems as complex as biological molecules. However, for higher excitation levels of ～200 GW/cm(2), there is a dramatic change in photophysics that leads to multiphoton generated photoproducts unrelated to the target isomerization reaction channel and drastically changes the observed isomerization kinetics that appears, in particular, as a red shift of the transient spectra. These results explain the apparent contradictions of the work by Florean et al. [Proc. Natl. Acad. Sci. U.S.A. 106, 10896 (2009)] in the high intensity regime. We are able to show that the difference in observations and interpretation is due to artifacts associated with additional multiphoton-induced photoproducts. At the proper monitoring wavelengths, coherent control in the high intensity regime is clearly observable. The present work highlights the importance of conducting coherent control experiments in the low intensity regime to access information on quantum interference effects along specific reaction coordinates.     

Pure optical dephasing dynamics in semiconducting single-walled carbon nanotubes

We report a detailed study of ultrafast exciton dephasing processes in semiconducting single-walled carbon nanotubes employing a sample highly enriched in a single tube species, the (6,5) tube. Systematic measurements of femtosecond pump-probe, two-pulse photon echo, and three-pulse photon echo peak shift over a broad range of excitation intensities and lattice temperature (from 4.4 to 292 K) enable us to quantify the timescales of pure optical dephasing (T(2)(*)), along with exciton-exciton and exciton-phonon scattering, environmental effects as well as spectral diffusion. While the exciton dephasing time (T(2)) increases from 205 fs at room temperature to 320 fs at 70 K, we found that further decrease of the lattice temperature leads to a shortening of the T(2) times. This complex temperature dependence was found to arise from an enhanced relaxation of exciton population at lattice temperatures below 80 K. By quantitatively accounting the contribution from the population relaxation, the corresponding pure optical dephasing times increase monotonically from 225 fs at room temperature to 508 fs at 4.4 K. We further found that below 180 K, the pure dephasing rate (1/T(2)(*)) scales linearly with temperature with a slope of 6.7 ± 0.6 μeV/K, which suggests dephasing arising from one-phonon scattering (i.e., acoustic phonons). In view of the large dynamic disorder of the surrounding environment, the origin of the long room temperature pure dephasing time is proposed to result from reduced strength of exciton-phonon coupling by motional narrowing over nuclear fluctuations. This consideration further suggests the occurrence of remarkable initial exciton delocalization and makes nanotubes ideal to study many-body effects in spatially confined systems.     

Infrared Femtosecond Laser Preionization in Secondary Ion Mass Spectrometry of Silver Surface

An alternative secondary ion mass spectrometry utilizing laser preionization is introduced. The native Ag sample surface is first irradiated with laser pulse (100 fs duration, 10(10)-10(11) W/cm(2) intensity, 1240 nm wavelength) and subsequently bombarded with primary ions (Bi(3)(+), 10 ns duration, 25 keV energy). Multiple correlation patterns are observed in the mass spectra, confirming the mutual laser-secondary ion mass spectrometry (SIMS) interplay in the preionization mechanism. The Ag(+), C(3)H(5)(+), C(3)H(5)O(3)(+), and AgOH(+), C(4)H(5)O(4)(+) are observed with the shallow and steep increasing of intensities at 1.3?×?10(11) W/cm(2) and 1.5?×?10(11) W/cm(2), respectively. Two ionization mechanisms are identified, the ion sputtering regime for intensities of less than 1.4?×?10(11) W/cm(2) and the multiphoton ionization at higher intensities. The Ag saturation intensity obtained from fitting is 2.4?×?10(13) W/cm(2), close to the one reported for postionization. The proposed preionization approach might eliminate the need for high peak power/high intensity laser source and, moreover, the experiment geometry ensures that large areas of the sample are affected by the laser beam.     

Studies of Photoionization of CF3 I by Femtosecond Time-resolved Mass Spectrometry

The multi-photon dissociative photoionization dynamics of CF3I has been studied with femtosecond two-color pump-probe time of flight mass spectra at a pump pulse of 265 nm and a probe pulse of 398 nm. The life constants of CF3I+ and its fragment ions CF3+ and I+ are obtained as (96±7), (198±130) and (167±6)fs, respectively. The multi-photon dynamics leading to these ions differ. CF3I+ corresponds to a (1+2′) transition with one-photon pump excitation to the A band of CF3I. CF3+ are mainly formed by a tow-photon probe excitation to the CF3+ with subsequent dissociation of parent ions. I+ are produced in (2+2′) combined with (1+1′+2′) process. The results provide information on the multi-photon pathways involved.     

Ultrafast dynamics in thiophene investigated by femtosecond pump probe photoelectron spectroscopy and theory

A hybrid of a time-of-flight mass spectrometer and a time-of-flight "magnetic-bottle type" photoelectron (PE) spectrometer is used for fs pump-probe investigations of the excited state dynamics of thiophene. A resonant two-photon ionization spectrum of the onset of the excited states has been recorded with a tunable UV laser of 190 fs pulse width. With the pump laser set to the first intense transition we find by UV probe ionization first a small time shift of the maxima in the PE spectrum and then a fast decay to a low constant intensity level. The fitted time constants are 80+/-10 fs, and 25+/-10 fs, respectively. Theoretical calculations show that upon geometry relaxation the electronic state order changes and conical intersections between excited states exist. We use the vertical state order S1, S2, S3 to define the terms S1, S2, and S3 for the characterization of the electron configuration of these states. On the basis of our theoretical result we discuss the electronic state order in the UV spectra and identify in the photoelectron spectrum the origin of the first cation excited state D1. The fast excited state dynamics agrees best with a vibrational dynamics in the photo-excited S1 (80+/-10 fs) and an ultrafast decay via a conical intersection, presumably a ring opening to the S3 state (25+/-10 fs). The subsequently observed weak constant signal is taken as an indication, that in the gas phase the ring-closure to S0 is slower than 50 ps. An ultrafast equilibrium between S1 and S2 before ring opening is not supported by our data.     

DESIGN AND PERFORMANCE OF THE OKAZAKI LARGE SPECTROGRAPH FOR PHOTOBIOLOGICAL RESEARCH

A computer-operated spectrograph was recently built at Okazaki, Japan. Different specimens can be placed on a horseshoe-shaped focal curve (10 m long) covering a wavelength range of 250 to 1000 nm so they can be irradiated simultaneously. The linear dispersion is about 0.8 nm/cm. The photon fluence rate on the focal curve is 5 x 10¹⁵. photons x cm^-2 x s^-1 at 300nm and 1 x 10¹⁶ photons x cm^-2 x s^-1 at 600 and at 900 nm. The spectral half width is 5.5 nm or less on the focal curve. The stray light content is about 10^-5 of the main peak at the peak wavelength ± 100 nm. Specimens are set in microcomputer-controlled threshold boxes so that wavelengths, photon fluence rates, photon fluences and timing of irradiations are controlled automatically according to a pre-programmed schedule. An optical fiber system is also provided for remote irradiations.     

Long-lived coherent oscillations of the femtosecond transients in cyanobacterial photosystem I

The pulsed excitation of electronic levels coupled to specific nuclear modes by a 26 fs laser pulse at 706 nm creates a wavepacket in the nuclear space of photopystem I (PS I) of Synechocystis sp. strain PCC 6803 both in the ground state and in the one-exciton manifold. Fourier transform of transient decay curves shows several low frequency peaks. The most prominent Power Spectral Density (PSD) peaks are at omega = 49 cm(-1) and omega = 88 cm(-1). The peculiarity of the coherent wavepacket in the PS I of S. sp. strain PCC 6803 is the unique, long-lived 49 cm(-1) and 88 cm(-1) oscillations with decay times up to 10 ps. It was suggested that such a long-lived coherence is determined by a contribution of the ground state wavepacket. The dependence of these two PSD peaks on the probe wavelength resembles the profile of the transient absorption spectra of PS I. The pump-probe signal in the Soret region reflects the dynamics of the ground state wavepacket created by pulsed excitation of the Q(y)-band. It was shown that the multimode Brownian oscillator model allows a quantitative fit of the oscillatory patterns of the pump-probe signal to be obtained.     

Wavelength-dependent Photodissociation Dynamics of Benzaldehyde

The ultrafast dynamics of benzaldehyde upon 260, 271, 284, and 287 nm excitations have been studied by femtosecond pinup-probe time-of-flight mass spectrometry. A bi-exponential decay component model was applied to fit the transient profiles of benzaldehyde ions and fragment ions. At the S2 origin, the first decay of the component was attributed to the internal conversion to the high vibrational levels of S1 state. Lifetimes of the first component decreased with increasing vibrational energy, due to the influence of high density of the vibrational levels. The second decay was assigned to the vibrational relaxation of the S1 whose lifetime was about 600 fs. Upon 287 nm excitation, the first decay became ultra-short （-56 fs） which was taken for the intersystem cross from S1 to T2, while the second decay component was attributed to the vibrational relaxation. The pump-probe transient of fragment was also studied with the different probe intensity at 284 nm pump.     

Delocalized electronic behavior observed in transition metal oxide clusters under strong-field excitation

Heterogeneously composed clusters are exposed to intensity resolved, 100 fs laser pulses to reveal the energy requirements for the production of the high charge states of both metal and nonmetal ions. The ionization and fragmentation of group V transition metal oxide clusters are here examined with laser intensities ranging nearly four orders in magnitude (～3 × 10(11) W/cm(2) to ～2 × 10(15) W/cm(2)) at 624 nm. The ionization potentials of the metal atoms are measured using both multiphoton ionization and tunneling ionization models. We demonstrate that the intensity selective scanning method can be utilized to measure the low ionization potentials of transition metals (～7 eV). The high charge states demonstrate an enhancement in ionization that is three orders of magnitude lower in laser intensity than predicted for the atomic counterparts. Finally, the response from the various metals and the oxygen is compared to elucidate the mechanism of enhanced ionization that is observed. Specifically, the sequence of ion appearances demonstrates delocalized electron behavior over the entire cluster.     

Single-Shot Gas-Phase Thermometry by Time-to-Frequency Mapping of Coherence Dephasing

We demonstrate a single-beam coherent anti-Stokes Raman scattering (CARS) technique for gas-phase thermometry that assesses the species-specific local gas temperature by single-shot time-to-frequency mapping of Raman-coherence dephasing. The proof-of-principle experiments are performed with air in a temperature-controlled gas cell. Impulsive excitation of molecular vibrations by an ultrashort pump/Stokes pulse is followed by multipulse probing of the 2330 cm(-1) Raman transition of N(2). This sequence of colored probe pulses, delayed in time with respect to each other and corresponding to three isolated spectral bands, imprints the coherence dephasing onto the measured CARS spectrum. For calibration purposes, the dephasing rates are recorded at various gas temperatures, and the relationship is fitted to a linear regression. The calibration data are then used to determine the gas temperature and are shown to provide better than 15 K accuracy. The described approach is insensitive to pulse energy fluctuations and can, in principle, gauge the temperature of multiple chemical species in a single laser shot, which is deemed particularly valuable for temperature profiling of reacting flows in gas-turbine combustors.     

Alignment, vibronic level splitting, and coherent coupling effects on the pump-probe polarization anisotropy

The pump-probe polarization anisotropy is computed for molecules with a nondegenerate ground state, two degenerate or nearly degenerate excited states with perpendicular transition dipoles, and no resonant excited-state absorption. Including finite pulse effects, the initial polarization anisotropy at zero pump-probe delay is predicted to be r(0) = 3/10 with coherent excitation. During pulse overlap, it is shown that the four-wave mixing classification of signal pathways as ground or excited state is not useful for pump-probe signals. Therefore, a reclassification useful for pump-probe experiments is proposed, and the coherent anisotropy is discussed in terms of a more general transition dipole and molecular axis alignment instead of experiment-dependent ground- versus excited-state pathways. Although coherent excitation enhances alignment of the transition dipole, the molecular axes are less aligned than for a single dipole transition, lowering the initial anisotropy. As the splitting between excited states increases beyond the laser bandwidth and absorption line width, the initial anisotropy increases from 3/10 to 4/10. Asymmetric vibrational coordinates that lift the degeneracy control the electronic energy gap and off-diagonal coupling between electronic states. These vibrations dephase coherence and equilibrate the populations of the (nearly) degenerate states, causing the anisotropy to decay (possibly with oscillations) to 1/10. Small amounts of asymmetric inhomogeneity (2 cm(-1)) cause rapid (130 fs) suppression of both vibrational and electronic anisotropy beats on the excited state, but not vibrational beats on the ground electronic state. Recent measurements of conical intersection dynamics in a silicon napthalocyanine revealed anisotropic quantum beats that had to be assigned to asymmetric vibrations on the ground electronic state only [Farrow, D. A.; J. Chem. Phys. 2008, 128, 144510]. Small environmental asymmetries likely explain the observed absence of excited-state asymmetric vibrations in those experiments.     

Fourth-order Raman spectroscopy of wide-band gap materials

Low-frequency surface vibrations were observed on a rutile TiO(2)(110) surface covered with trimethyl acetate (TMA) by using fourth-order Raman spectroscopy. The TMA-covered surface interfaced to air was irradiated with 18-fs light at a wavelength of 630 nm. A pump pulse excited vibrational coherence of Raman-active modes and a probe pulse interacts with the coherently excited surface to generate second harmonic light (315 nm), the intensity of which oscillated as a function of the pump-probe delay. Four bands were recognized at 180, 357, 444, and 826 cm(-1) in the Fourier transformation spectrum of the oscillation and assigned to bulk phonons modified by the presence of the surface boundary condition. The Raman transition for the pump was nonresonant to the band gap excitation of TiO(2), as evidenced by the oscillation phase relative to the pump irradiation and by the oscillation amplitude as a function of the pump power. The observable range of this surface-selective spectroscopy is extended to wide-band gap materials on which one-photon resonance enhancement of the Raman-pump efficiency cannot be expected.     

Ultrafast S1 to S0 internal conversion dynamics for dimethylnitramine through a conical intersection

Electronically nonadiabatic processes such as ultrafast internal conversion (IC) from an upper electronic state (S(1)) to the ground electronic state (S(0)) though a conical intersection (CI), can play an essential role in the initial steps of the decomposition of energetic materials. Such nonradiative processes following electronic excitation can quench emission and store the excitation energy in the vibrational degrees of freedom of the ground electronic state. This excess vibrational energy in the ground electronic state can dissociate most of the chemical bonds of the molecule and can generate stable, small molecule products. The present study determines ultrafast IC dynamics of a model nitramine energetic material, dimethylnitramine (DMNA). Femtosecond (fs) pump-probe spectroscopy, for which a pump pulse at 271 nm and a probe pulse at 405.6 nm are used, is employed to elucidate the IC dynamics of this molecule from its S(1) excited state. A very short lifetime of the S(1) excited state (～50 ± 16 fs) is determined for DMNA. Complete active space self-consistent field (CASSCF) calculations show that an (S(1)/S(0))(CI) CI is responsible for this ultrafast decay from S(1) to S(0). This decay occurs through a reaction coordinate involving an out-of-plane bending mode of the DMNA NO(2) moiety. The 271 nm excitation of DMNA is not sufficient to dissociate the molecule on the S(1) potential energy surface (PES) through an adiabatic NO(2) elimination pathway.     

Fragmentation dynamics of fullerenes in intense femtosecond-laser fields: loss of small neutral fragments on a picosecond time scale

The fragmentation dynamics of C60 irradiated with intense femtosecond laser pulses is studied with one-color pump-probe spectroscopy. Small neutral fragments (C, C2, and C3) are formed by an 800-nm pump pulse which are then postionized by a delayed probe pulse. The respective ion signals detected by the time-of-flight mass spectrometry dramatically increase on a time scale of 10-20 ps.     

Protoporphyrin IX fluorescence kinetics in UV-induced tumours and normal skin of hairless mice after topical application of 5-aminolevulinic acid methyl ester

Accumulation of protoporphyrin IX (PpIX) was investigated in normal skin and UV-induced tumours in hairless mice after topical application of a cream containing 2, 8 or 16% of 5-aminolevulinic acid methyl ester (ALA-Me). Higher levels of PpIX were measured in tumours compared to normal skin. The maximal amount of PpIX was reached at 1.5, 3 and 4 h after 2, 8 and 16% ALA-Me application, respectively. Higher tumour to normal skin PpIX fluorescence ratios were measured after application of 8 and 16% ALA-Me than after application of 2%. After irradiation with a broad spectrum of visible light from a slide projector, more than 90% of PpIX was bleached by fluences of 36 and 48 J/cm2, at fluence rates of 10 and 40 mW/cm2 respectively. At these fluences, the PpIX photobleaching rate was significantly higher (P<0.05) in normal mouse skin than in tumours. In addition, for a given fluence, more PpIX was photobleached at the lower fluence rate (10 mW/cm2) than at the higher fluence rate (40 mW/cm2) in normal skin (P<0.001) as well as in tumours (P<0.05) after exposure to 24 J/cm2 of light. In conclusion, the highest tumour to normal skin PpIX ratio was observed 3 h after application of 8% ALA-Me, suggesting that light exposure should be performed at this time in order to achieve an optimal PDT effect in this tumour model.     

Ultrafast energy transfer in light-harvesting chlorosomes from the green sulfur bacterium Chlorobium tepidum

Two independent pump-probe techniques were used to study the antenna energy transfer kinetics of intact chlorosomes from the green sulfur bacterium Chlorobium tepidum with femtosecond resolution. The isotropic kinetics revealed by one-color experiments in the BChl c antenna were inhomogeneous with respect to wavelength. Multiexponential analyses of the photobleaching/stimulated emission (PB/SE) decay profiles typically yielded (apart from a approximately 10 fs component that may stem from the initial coherent oscillation) components with lifetimes 1-2 ps and several tens of ps. The largest amplitudes for the latter component occur at 810 nm, the longest wavelength studied. Analyses of most two-color pump-probe profiles with the probe wavelength red-shifted from the pump wavelength yielded no PB/SE rise components. PB/SE components with approximately 1 ps risetime were found in 790 --> 810 and 790 --> 820 nm profiles, in which the probe wavelength is situated well into the BChl a absorption region. A 760 --> 740 nm uphill two-color experiment yielded a PB/SE component with 4-6 ps risetime. Broadband absorption difference spectra of chlorosomes excited at 720 nm (in the blue edge of the 746 nm BChl c Qy band) exhibit approximately 15 nm red-shifting of the PB/SE peak wavelength during the first several hundred fs. Analogous spectra excited at 760 nm (at the red edge) show little dynamic spectral shifting. Our results suggest that inhomogeneous broadening and spectral equilibration play a larger role in the early BChl c antenna kinetics in chlorosomes from C. tepidum than in those from C. aurantiacus, a system studied previously. As in C. aurantiacus, the initial one-color anisotropies r(0) for most BChl c wavelengths are close to 0.4. The corresponding residual anisotropies r(infinity) are typically 0.19-0.25, which is much lower than found in C. aurantiacus (> or = 0.35); the transition moment organization is appreciably less collinear in the BChl c antenna of C. tepidum. However, the final one-color anisotropies at 789 and 801 nm are approximately 0 and 0.09 respectively, and the final anisotropy in time 780 --> 800 nm experiment is approximately -0.1. These facts indicate that the BChI a transition moments themselves exhibit some order, and are directed at an angle > 54.7 degrees on the average from the BChl c moments. The one-color profiles exhibit coherent oscillations at most wavelengths, including 800 nm; Fourier analyses of these oscillations frequently yield components with frequencies 70-80 and 130-140 cm-1.