UV Pump Research Articles

Photoinduced π−π* Band Gap Renormalization in Graphite

As is well-known, the character of the π orbitals is of paramount importance for the chemical properties of the carbon allotropes and their derived compounds. While at equilibrium the nature of these orbitals is well understood, their photoinduced nonequilibrium behavior is under investigation. Here, we demonstrate that when a UV-laser pulse excites a carrier density larger than 10% of the π* density of state in graphite, a renormalization of the π-π* band gap takes place. This result has been achieved by detecting the transient reflectivity and the associated decay time of an infrared probe following the excitation of a UV pump pulse tuned across the π-π* absorption resonance. The pump photon energy at which both the transient reflectivity and the decay time are maximum is downshifted by 500 meV with respect to the relative absorption maximum at equilibrium. This finding is interpreted as a transient π-π* band gap shrinking of similar magnitude, near the M point of the Brillouin zone.

Photoluminescence properties of Sr 3(VO 4) 2, Sr 2Y 2/3−yEu y(VO 4) 2 and Sr 2Y 2/3−zSm z(VO 4) 2

Photoluminescence properties of Sr 3(VO 4) 2, Sr 2Y 2/3− y Eu y (VO 4) 2 and Sr 2Y 2/3− z Sm z (VO 4) 2 ( y, z = 0, 0.1, 0.2, and 0.3) samples have been investigated. The self-activated Sr 3(VO 4) 2 sample shows the activation and emission bands due to the charge transfer transition of VO 4 3 - . While Sr 2+ is partially substituted by Y 3+, Sr 2Y 2/3 (VO 4) 2 can be used as the good host material for Eu 3+ and Sm 3+ activator-ions. The Eu 3+ and Sm 3+ ions doped Sr 2Y 2/3(VO 4) 2 can be effectively excited by the near UV light with wavelengths of 300 nm, and emit red light around 617 and 604 nm, respectively. Specially, for Sr 2Y 2/3− y Eu y (VO 4) 2, Y 3+ substitution quite enhances the emission intensity and quenching concentration of Eu 3+. For example, Sr 2Y 0.56Eu 0.1(VO 4) 2 has higher emission intensity than Sr 2.85Eu 0.1(VO 4) 2 without Y 3+, and even much higher than Y 1− x Eu x VO 4 which has been extensively used. Our results suggest that Sr 2Y 2/3− y Eu y (VO 4) 2 samples may be good candidate for a new phosphor to convert the UV pump light into visible wavelength bands.

Resolution of strongly competitive product channels with optimal dynamic discrimination: Application to flavins

Fundamental molecular selectivity limits are probed by exploiting laser-controlled quantum interferences for the creation of distinct spectral signatures in two flavin molecules, erstwhile nearly indistinguishable via steady-state methods. Optimal dynamic discrimination (ODD) uses optimally shaped laser fields to transiently amplify minute molecular variations that would otherwise go unnoticed with linear absorption and fluorescence techniques. ODD is experimentally demonstrated by combining an optimally shaped UV pump pulse with a time-delayed, fluorescence-depleting IR pulse for discrimination amongst riboflavin and flavin mononucleotide in aqueous solution, which are structurally and spectroscopically very similar. Closed-loop, adaptive pulse shaping discovers a set of UV pulses that induce disparate responses from the two flavins and allows for concomitant flavin discrimination of ∼16σ. Additionally, attainment of ODD permits quantitative, analytical detection of the individual constituents in a flavin mixture. The successful implementation of ODD on quantum systems of such high complexity bodes well for the future development of the field and the use of ODD techniques in a variety of demanding practical applications.

Probing vibrational wave packets in molecular excited states

A time-dependent theoretical method is used to describe a UV pump–UV probe strategy to trace, at a femtosecond time scale, the motion of vibrational wave packets created in excited states of the hydrogen molecule by measuring single ionization probabilities. We use a spectral method to solve the time-dependent Schrodinger equation in full dimensionality, including correlation and all electronic and vibrational degrees of freedom. A pump pulse initially creates a vibrational wave packet in the intermediate electronic excited states of $$\hbox{H}_2$$ . The frequency of the probe is chosen to ionize the target leaving the ion in a bound vibrational state. By varying the time delay between pulses, non-dissociative single ionization is enhanced or suppressed. Energy differential ionization probabilities are reported and compared with a model based on the Franck–Condon approximation.

Full Optical Control of Holographic Gratings Realized in Composite Materials Containing Photosensitive Liquid Crystals

We report on the characterization of an all optical switching effect that takes place when a UV light beam irradiates an azo-POLICRYPS diffraction grating. This high quality polymeric structure is utilized to confine and stabilize a well aligned nematic liquid crystal (NLC) film doped with a small percentage of azo-NLC molecules. The optical control of the grating diffraction efficiency is obtained by exploiting the photochemical phase transition between the nematic and the isotropic phases of the liquid crystal, based on the photoisomerization of the azobenzene guest molecules. The switch ON-OFF of the grating diffraction efficiency is triggered by a UV pump laser and is investigated by means of a low power red probe beam.

Remarkable Site Difference of Vibrational Energy Relaxation in Benzene Dimer: Picosecond Time‐Resolved IR–UV Pump–Probe Spectroscopy

Benzene dimer (Bz2) is one of the most fundamental molecular systems and shows intriguing properties attributed to aromaticaromatic weak interactions. The structure of Bz2 in the ground electronic state (S0) has been extensively studied experimentally and theoretically. Several experimental studies on Bz2 such as the molecular-beam electric deflection 1(b)] and Raman-UV double resonance and UV-UV hole-burning spectroscopy strongly suggested that the two benzene (Bz) molecules are not symmetrically equivalent. In addition, recent high level ab initio calculations suggested that a floppy T-shaped structure is a global minimum, although there are several possible isomers in nearly isoenergetic region. The rotational constant obtained from the Fourier-transform microwave spectrum agreed well with the predicted value for the T-shaped form. An important subject strongly related to the structure is the vibrational dynamics. In the T-shaped Bz2, the two Bz molecules are at different symmetrical conditions; one is at the Stem-site, and the other is at the Top-site. Therefore, the rate constant as well as the process of the vibrational energy relaxation of the two molecules will be different. In this sense, the study of a site-specified vibrational energy relaxation will provide us with a more detailed picture of the structure and vibrational dynamics of Bz2. Several studies on the vibrational energy relaxation of Bz2 have been reported. Lee and co-workers estimated the lifetime of the CH stretching region to be ~3 ps based on the band-width measurement of the IR spectrum. Felker and co-workers suggested that the 2 (CH stretching, Wilson’s numbering) level of the Stem-site Bz relaxes faster than that of the Top-site Bz, according to the difference in the band-width of the Raman spectra. However, in both cases the measured band-width was not that of a single rovibrational level. It is also not clear how the band-width is related to the vibrational energy relaxation dynamics, such as intramolecular (intracluster) vibrational energy redistribution (IVR) and vibrational predissociation (VP). Moreover, there has been no report on the time-resolved spectroscopy for the site-specified Bz2. Here we study the dynamics of the vibrational energy relaxation of the T-shaped Bz2 in the CH stretching energy region by using picosecond time-resolved IR-UV pump-probe spectroscopy. It is known that for the isolated Bz monomer having a D6h symmetry the IR active CH vibration (20) with the e1u symmetry is anharmonically coupled with the 8+19 and 1+6+19 Figure 1. Energy level diagram of isotope-substituted benzene dimers [h(Stem)d(Top) and d(Stem)h(Top)] and an excitation scheme. The dimers are vibrationally excited to the CH stretching region by a picosecond IR pulse, and the time evolution is probed by a picosecond UV pulse. The h*(Stem) and h*(Top) benzene components (an asterisk represents the vibrationally excited site) show different vibronic structures. Thus, the vibrationally excited dimers can be separately monitored by a tunable picosecond UV pulse. In this figure, VER means vibrational energy relaxation.

Remarkable Site Difference of Vibrational Energy Relaxation in Benzene Dimer: Picosecond Time‐Resolved IR–UV Pump–Probe Spectroscopy

Remarkable Site Difference of Vibrational Energy Relaxation in Benzene Dimer: Picosecond Time‐Resolved IR–UV Pump–Probe Spectroscopy

Novel opportunities for time-resolved absorption spectroscopy at the X-ray free electron laser

Time-dependent X-ray Absorption Spectroscopy (XAS) measurements of chemical reaction dynamics have a time resolution which is limited by: (a) the speed and efficiency of the reaction initiation; (b) the duration of the X-ray pulses used for the measurement; and (c) the brightness of the X-ray source. X-Ray Free Electron Lasers (XFEL), which will deliver 20-100 fs pulses of X-rays, with a peak brightness which is 10(10) times that of a synchrotron, will alleviate limitations (b) and (c). Furthermore, by including a synchronized source of UV, visible, IR or THz pump radiation, the XFEL will contribute to the solution of limitation (a). The present article describes the XFEL operating principle and the generic design of an XFEL facility, emphasizing the features of particular interest to the XAS investigator.

Sm 3+-doped polymer optical waveguide amplifiers

Trivalent samarium ion (Sm 3+) doped SU8 polymer materials were synthesized and characterized. Intense red emission at 645 nm was observed under UV laser light excitation. Spectroscopic investigations show that the doped materials are suitable for realizing planar optical waveguide amplifiers. About 100 μm wide multimode Sm 3+-doped SU8 channel waveguides were fabricated using a simple UV exposure process. At 250 mW, 351 nm UV pump power, a signal enhancement of ∼7.4 dB at 645 nm was obtained for a 15 mm long channel waveguide.

Vibrational cooling in the liquid phase studied by ultrafast investigations of cycloheptatriene

Solvent-mediated vibrational relaxation in UV pump–probe experiments has been studied using cycloheptatriene (CHT) and its perdeuterated counterpart (CHTd8) as model systems. Transient spectra containing absolute ε(t) values are obtained by a modified Sulzer–Wieland ansatz and a global fit function for vibrational relaxation is proposed.

Relaxation dynamics of NH stretching vibrations of 2-aminopyridine and its dimer in a supersonic beam

Picosecond time-resolved IR-UV pump-probe spectroscopy has been carried out to elucidate vibrational energy relaxation (VER) of the NH stretching vibrations of 2-aminopyridine monomer (2AP) and dimer [(2AP)(2)] in supersonic beams. In bare 2AP, intramolecular vibrational energy redistribution (IVR) of the NH vibrations is described by the two-bath mode model, in which the initial vibrational energy flows to the doorway states rapidly (6.5 ps) and then dissipates into the dense bath states with a time constant of approximately 20 ps. No clear difference was observed in the IVR lifetime between the symmetric and asymmetric NH(2) stretch modes. In (2AP)(2), IVR and vibrational predissociation (VP) were involved in VER. It was found that the rate constants of both IVR and VP of the hydrogen-bonded NH stretching vibration are larger than those of the free NH.

Quantum control of the photoinduced Wolff rearrangement of diazonaphthoquinone in the condensed phase

A shaped UV pump–MIR probe setup is employed for quantum control of the photoinduced Wolff rearrangement reaction of diazonaphthoquinone (DNQ) dissolved in methanol, yielding a ketene photoproduct. Time-resolved vibrational spectroscopy is a well-suited tool to monitor a photoreaction in the liquid phase as the narrow vibrational lines allow the observation of structural changes. Especially in the mid-infrared region, marker modes originating from different photoproducts can be identified unambiguously providing suitable feedback signals for open-loop or closed-loop control schemes. We report an experiment where the initiation of a complicated structural change of a molecule, involving bond cleavage and rearrangement, in the liquid phase can be controlled and mechanistic insight is obtained. Single-parameter scans show that the molecule is sensitive to intrapulse dumping during the excitation. Adaptive optimizations lead to pulse structures which can be understood consistently with this dumping mechanism.

Ultrafast excited-state dynamics of RNA and DNA C tracts

The excited-state dynamics of the RNA homopolymer of cytosine and of the 18-mer (dC) 18 were studied by steady-state and time-resolved absorption and emission spectroscopy. At pH 6.8, excitation of poly(rC) by a femtosecond UV pump pulse produces excited states that decay up to one order of magnitude more slowly than the excited states formed in the mononucleotide cytidine 5′-monophosphate under the same conditions. Even slower relaxation is observed for the hemiprotonated, self-associated form of poly(rC), which is stable at acidic pH. Transient absorption and time-resolved fluorescence signals for (dC) 18 at pH 6.8 are similar to ones observed for poly(rC) near pH 4, indicating that hemiprotonated structures are found in DNA C tracts at neutral pH. In both systems, there is evidence for two kinds of emitting states with lifetimes of ∼100 ps and slightly more than 1 ns. The former states are responsible for the bulk of emission from the hemiprotonated structures. Evidence suggests that slow electronic relaxation in these self-complexes is the result of vertical base stacking. The similar signals from RNA and DNA C tracts suggest a common base-stacked structure, which may be identical with that of i-motif DNA.

Resonant uv pump-probe spectroscopy of dipicolinic acid via impulsive excitation

We present experimental evidence of coherent wave packet motion in dipicolinic acid $({\mathrm{C}}_{7}{\mathrm{H}}_{5}\mathrm{N}{\mathrm{O}}_{4})$ which is an important marker molecule for bacterial spores. Resonant impulsive excitation is achieved by applying a uv pump pulse ($267\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $16\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$) which has a duration that is shorter than the vibrational period of the molecules. The resulting dynamics is then probed with a weaker pulse of the same width and frequency. Evidence of the important ``fingerprint'' region for this molecule (between $1000\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and $1500\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$) is found in the transient absorption of the probe. We present simulations of the pump-probe experiment, based on the Liouville equation for the density matrix, and predict the optimal pulse width and detuning.

Ultrasensitive ultraviolet-visible 20fs absorption spectroscopy of low vapor pressure molecules in the gas phase

We describe an ultrasensitive pump-probe spectrometer for transient absorption measurements in the gas phase and in solution. The tunable UV pump and the visible (450-740 nm) probe pulses are generated by two independently tunable noncollinear optical parametric amplifiers, providing a temporal resolution of 20 fs. A homebuilt low gain photodetector is used to accommodate strong probe pulses with a shot noise significantly lower than the overall measurement noise. A matched digitizing scheme for single shot analysis of the light pulses at kilohertz repetition rates that minimizes the electronic noise contributions to the transient absorption signal is developed. The data processing scheme is optimized to yield best suppression of the laser excess noise and thereby transient absorbance changes down to 1.1 x 10(-6) can be resolved. A collinear focusing geometry optimized for a 50 mm interaction length combined with a heatable gas cell allows us to perform measurements on substances with low vapor pressures, e.g., on medium sized molecules which are crystalline at room temperature. As an application example highlighting the capability of this instrument, we present the direct time-domain observation of the ultrafast excited state intramolecular proton transfer of 2-(2(')-hydroxyphenyl)benzothiazole in the gas phase. We are able to compare the resulting dynamics in the gas phase and in solution with a temporal precision of better than 5 fs.

Picosecond IR−UV Pump−Probe Study on the Vibrational Relaxation of Phenol−Ethylene Hydrogen-Bonded Cluster: Difference of Relaxation Route/Rate between the Donor and the Acceptor Site Excitations

Picosecond time-resolved IR-UV pump-probe spectroscopy has been performed to study intracluster vibrational energy redistribution (ICVR) and vibrational predissociation (VP) for the OH and CH stretch vibrations of phenol-ethylene hydrogen-bonded cluster. The transient UV spectra after the picosecond IR pulse excitation of these modes were observed by 1+1 REMPI via S(1) with a picosecond UV pulse. We have focused on the difference of the energy flow routes and their rates between the donor (phenol) and the acceptor (ethylene) site. Though the transient UV spectra showed a similar broad feature for all the vibrations examined, the time profiles exhibited a remarkable site dependence, as well as substantial mode dependence. Especially, we found a large difference in the early stage of the IVR evolution and the rates, whereas the VP rates were very similar.

Cover Picture: Real‐Time Observation of Ionization‐Induced Hydrophobic→Hydrophilic Switching (Angew. Chem. Int. Ed. 38/2005)

The intermolecular binding motif of the argon atom in a phenol⋅⋅⋅Ar dimer has been monitored on the picosecond timescale by time‐resolved UV–UV(pump)–IR(probe) ionization depletion spectroscopy. The cover picture shows the dynamic process, which involves a shift of the initially π‐bound Ar atom to a hydrophilic, hydrogen‐bound site. For more details, see the Communication by M. Fujii, K. Müller‐Dethlefs, and co‐workers on page 6149 ff.

Transient two-dimensional infrared spectroscopy: exploring the polarization dependence.

We present a general expression for the polarization dependence of transient two-dimensional IR spectroscopy (T2D-IR), a technique designed to measure 2D-IR spectra of transient species. T2D-IR is a UV pump narrowband-IR-pump broadband-IR-probe experiment of fifth order in the laser field which involves up to three different transition dipole moments. The UV pulse adds an additional degree of freedom in polarization as compared to 2D-IR spectroscopy and increases the versatility of signal manipulation and the potential structural information content of the signals. The polarization conditions leading to a maximum of structural information are discussed. Important special cases of polarization conditions are formulated. The application of polarization selectivity is demonstrated for different types of T2D-IR experiments on photo triggered metal-to-ligand charge transfer in the model system [Re(CO)(3)(dmbpy)Cl].

Picosecond IR-UV pump-probe spectroscopic study of the dynamics of the vibrational relaxation of jet-cooled phenol. I. Intramolecular vibrational energy redistribution of the OH and CH stretching vibrations of bare phenol.

The intramolecular vibrational energy redistribution (IVR) of the OH stretching vibration of jet-cooled phenol-h6 (C6H8OH) and phenol-d8 (C6D8OH) in the electronic ground state has been investigated by picosecond time-resolved IR-UV pump-probe spectroscopy. The OH stretching vibration of phenol was excited with a picosecond IR laser pulse, and the subsequent temporal evolutions of the initially excited level and the redistributed ones due to the IVR were observed by multiphoton ionization detection with a picosecond UV pulse. The IVR lifetime for the OH stretch vibration of phenol-h6 was determined to be 14 ps, while that of the OH stretch for phenol-d8 was found to be 80 ps. This remarkable change of the IVR rate constant upon the dueteration of the CH groups strongly suggests that the "doorway states" for the IVR from the OH level would be the vibrational states involving the CH stretching modes. We also investigated the IVR rate of the CH stretching vibration for phenol-h6. It was found that the IVR lifetime of the CH stretch is less than 5 ps. The fast IVR is described by the strong anharmonic resonance of the CH stretch with many other combinations or overtone bands.

Picosecond IR-UV pump-probe spectroscopic study of the dynamics of the vibrational relaxation of jet-cooled phenol. II. Intracluster vibrational energy redistribution of the OH stretching vibration of hydrogen-bonded clusters.

A picosecond time-resolved IR-UV pump-probe spectroscopic study has been carried out for investigating the intracluster vibrational energy redistribution (IVR) and subsequent dissociation of hydrogen-bonded clusters of phenol (C6H5OH) and partially deuterated phenol (C6D5OH, phenol-d5) with various solvent molecules. The H-bonded OH stretching vibration was pumped by a picosecond IR pulse, and the transient S1-S0 UV spectra from the pumped level as well as the redistributed levels were observed with a picosecond UV laser. Two types of hydrogen-bonded clusters were investigated with respect to the effect of the H-bonding strength on the energy flow process: the first is of a strong "sigma-type H-bond" such as phenol-(dimethyl ether)(n=1) and phenol dimer, and the second is phenol-(ethylene)(n=1) having a weak "pi-type H-bond." It was found that the population of the IR-pumped OH level exhibits a single-exponential decay, whose rate increases with the H-bond strength. On the other hand, the transient UV spectrum due to the redistributed levels showed a different time evolutions at different monitoring UV frequency. From an analysis of the time profiles of the transient UV spectra, the following three-step scheme has been proposed for describing the energy flow starting from the IVR of the initially excited H-bonded OH stretching level to the dissociation of the H bond. (1) The intramolecular vibrational energy redistribution takes place within the phenolic site, preparing a hot phenol. (2) The energy flows from the hot phenol to the intermolecular vibrational modes of the cluster. (3) Finally, the hydrogen bond dissociates. Among the three steps, the rate constant of the first step was strongly dependent on the H-bond strength, while the rate constants of the other two steps were almost independent of the H-bond strength. For the dissociation of the hydrogen bond, the observed rate constants were compared with those calculated by the Rice, Ramsperger, Kassel, and Marcus model. The result suggests that dissociation of the hydrogen bond takes place much faster than complete energy randomization within the clusters.