Ultrafast UV Research Articles

Photochemistry of 2-Naphthoyl Azide. An Ultrafast Time-Resolved UV–Vis and IR Spectroscopic and Computational Study

The photochemistry of 2-naphthoyl azide was studied in various solvents by femtosecond time-resolved transient absorption spectroscopy with IR and UV-vis detection. The experimental findings were interpreted with the aid of computational studies. Using polar and nonpolar solvents, the formation and decay of the first singlet excited state (S(1)) was observed by both time-resolved techniques. Three processes are involved in the decay of the S(1) excited state of 2-naphthoyl azide: intersystem crossing, singlet nitrene formation, and isocyanate formation. The lifetime of the S(1) state decreases significantly as the solvent polarity increases. In all solvents studied, isocyanate formation correlates with the decay of the azide S(1) state. Nitrene formation correlates with the decay of the relaxed S(1) state only upon 350 nm excitation (S(0) → S(1) excitation). When S(n) (n ≥ 2) states are populated upon excitation (λ(ex) = 270 nm), most nitrene formation takes place within a few picoseconds through the hot S(1) and higher singlet excited states (S(n)) of 2-naphthoyl azide. The data correlate with the results of electron density difference calculations that predict nitrene formation from the higher-energy singlet excited states, in addition to the S(1) state. For all of these experiments, no recovery of the ground state was observed up to 3 ns after photolysis, which indicates that both internal conversion and fluorescence have very low efficiencies.

Photodynamical simulations of cytosine: characterization of the ultrafast bi-exponential UV deactivation

Deactivation of UV-excited cytosine is investigated by non-adiabatic dynamics simulations, optimization of conical intersections, and determination of reaction paths. Quantum chemical calculations are performed up to the MR-CISD level. Dynamics simulations were performed at multiconfigurational level with the surface hopping method including four electronic states. The results show the activation of four distinct reaction pathways at two different subpicosecond time scales and involving three different conical intersections. Most trajectories relax to a minimum of the S(1) state and deactivate with a time constant of 0.69 ps mainly through a semi-planar conical intersection along the n(O)π* surface. A minor fraction deactivate along ππ* regions of the S(1) surface. Sixteen percent of trajectories do not relax to the minimum and deactivate with a time constant of only 13 fs.

Ultrafast UV−Vis and IR Studies of p-Biphenylyl Acetyl and Carbomethoxy Carbenes

The photochemistry of a p-biphenylyl diazo ester (BpCN2CO2CH3) and diazo ketone (BpCN2COCH3) were studied by ultrafast time-resolved UV-vis and IR spectroscopies. The excited states of these diazo compounds were detected and found to decay with lifetimes of less than 300 fs. The diazo ester produces singlet carbene with greater quantum efficiency than the ketone analogue due to competing Wolff rearrangement (WR) in the excited state of the diazo ketone. Carbene BpCCO2CH3 has a singlet-triplet gap that is close to zero in cyclohexane, but the triplet is the ground state. The two spin states are in rapid equilibrium in this solvent relative to reaction with cyclohexane. There is (for a carbene) a slow rate of singlet to triplet intersystem crossing (isc) in this solvent because the orthogonal singlet must rotate to a higher energy orientation prior to isc. In acetonitrile and in dichloromethane BpCCO2CH3 has a singlet ground state. Ketocarbene BpCCOCH3 has a singlet ground state in cyclohexane, in dichloromethane, and in acetonitrile and decays by WR to form a ketene detected by ultrafast IR spectroscopy in these solvents. Ketocarbenes have more stable singlet states, relative to carbene esters, because of the superior conjugation of the filled hybrid orbital of the carbene with the pi system of the carbonyl group, the same factor that makes methyl ketones more acidic than the analogous esters. The rate of WR of BpCCOCH3 is faster in cyclohexane than in dichloromethane and acetonitrile because of intimate solute-solvent interactions between the empty p orbital of the carbene and nonbonding electron pairs of heteroatoms of the solvent. These interactions stabilize the carbene and retard the rate of WR.

Ultrafast UV photon echo peak shift and fluorescence up conversion studies of non-polar solvation dynamics

We present photon echo peak shift and femtosecond fluorescence up-conversion studies of non-polar solvation dynamics of a simple non-polar dye p-terphenyl in ethanol and cyclohexane, using excitation in the UV range at 290 nm. The UV fluorescence up-conversion experiments were combined with a polychromatic detection and the results highlight the high sensitivity of this approach to fully characterize the excited state dynamics of the dye. We also demonstrate the feasibility of UV photon echo and transient grating and its sensitivity for the detection of non-polar solvation dynamics by measuring the frequency correlation function of the dye in the ground state. While solvation dynamics in the picosecond regime is observed in ethanol, electronic coherence dephasing occurs on timescales faster than 100 fs in ethanol as well as in the non-polar solvent cyclohexane.

Primary photoprocesses in oxyblepharismin interacting with its native protein partner

The primary photoprocesses in the photoreceptor for the step-up photophobic response of the light-adapted cells of Blepharisma japonicum (OBIP, OxyBlepharismin bInding Protein) have been studied by ultrafast UV–vis transient spectroscopy. The results are rationalized in terms of heterogeneity of the OBIP sample. Two independent classes of chromoprotein are proposed: a “reactive” species, which presents a specific 680-nm band decaying in 4 and 56 ps and a “non-reactive” one, which behaves like the free chromophore (OxyBP) in solution. A bimolecular photooxidation of OxyBP in the presence of 1,4-benzoquinone was performed to record the absorption spectrum of the OxyBP radical cation. Comparison with reactive OBIP suggests that an electron transfer could be involved in the primary photoprocesses of OBIP and possibly trigger the sensory transduction chain of B. japonicum. In addition, the specificity of the chromophore–protein interaction was investigated by studying the artificial complex that OxyBP forms with human serum albumin (HSA). OxyBP–HSA happens to be spectroscopically much closer to free OxyBP than to OBIP. This highlights the specific nature of the interaction between OxyBP and its native protein partner and further supports the proposal that OBIP is the actual photoreceptor for the photophobic response of B. japonicum.

Insights into excited-state and isomerization dynamics of bacteriorhodopsin from ultrafast transient UV absorption

A visible-pump/UV-probe transient absorption is used to characterize the ultrafast dynamics of bacteriorhodopsin with 80-fs time resolution. We identify three spectral components in the 265- to 310-nm region, related to the all-trans retinal, tryptophan (Trp)-86 and the isomerized photoproduct, allowing us to map the dynamics from reactants to products, along with the response of Trp amino acids. The signal of the photoproduct appears with a time delay of approximately 250 fs and is characterized by a steep rise ( approximately 150 fs), followed by additional rise and decay components, with time scales characteristic of the J intermediate. The delayed onset and the steep rise point to an impulsive formation of a transition state on the way to isomerization. We argue that this impulsive formation results from a splitting of a wave packet of torsional modes on the potential surface at the branching between the all-trans and the cis forms. Parallel to these dynamics, the signal caused by Trp response rises in approximately 200 fs, because of the translocation of charge along the conjugate chain, and possible mechanisms are presented, which trigger isomerization.

Rapid Periodic Binding and Displacement of the Glucocorticoid Receptor during Chromatin Remodeling

An ultrafast UV laser crosslinking assay has provided novel insights into the progression of the SWI/SNF-mediated chromatin-remodeling reaction and transcription factor binding in real time. We demonstrate site-specific crosslinking between the glucocorticoid receptor (GR), the hSWI/SNF chromatin-remodeling complex, and the mouse mammary tumor virus (MMTV) promoter assembled in an array of correctly positioned nucleosomes. GR first demonstrates rapid binding to the promoter and then is actively displaced from the template during the remodeling reaction. This displacement reaction requires the hSWI/SNF complex and ATP, is specific to the nucleoprotein template, and is accompanied by a core histone rearrangement. The hSWI/SNF complex associates with random positions on the chromatin template in the absence of GR but is recruited specifically to the B/C region when GR is included. These results indicate that enhancement of hSWI/SNF-mediated factor accessibility, a hallmark of chromatin remodeling, is in some cases transient, reversible, and periodic.

Ultrafast UV pump/IR probe studies of C-H activation in linear, cyclic, and aryl hydrocarbons.

The photochemical C-H activation reactions of eta(3)-TpRh(CO)(2) (Tp = HB-Pz(3), Pz = 3,5-dimethylpyrazolyl) and CpRh(CO)(2) (Cp = C(5)H(5)) have been studied in a series of linear, cyclic, and aromatic hydrocarbon solvents on a femtosecond to microsecond time scale. These results have revealed that the structure of the hydrocarbon substrate affects the final C-H bond activation step, which is in accordance with the known preference of bond activation toward primary C-H sites. In the case of aromatic C-H activation, the reaction is divided into parallel channels involving sigma- and pi-solvated intermediates. Results for the analogous CpRh(CO)(2) molecule have shown that the coordination of the cyclopentadienyl ligand does not play a direct role in the dynamics of the reaction, in contrast to the C-H activation mechanism observed in eta(3)-TpRh(CO)(2) studies.

Ultrafast laser control of ionic-bond formation: ClF in argon solids

The formation of the two ionic products of Cl+F− versus Arn+F− is controlled by variation of the time delay between two ultrafast UV pulses. The Arn+F− exciplex is characterized by a fluorescence band at 355 nm and the Cl+F− product by the D′ emission at 420 nm. In this Tannor–Kosloff–Rice control scheme, the pump pulse dissociates ClF in a solid argon matrix. During the early dynamics in the matrix cage the F fragments have sufficient kinetic energy to closely approach the Ar atoms. Here a control pulse, which is delayed by typically 3 ps, induces the ionic bond Arn+F−. Energy loss by collisions with the cage leads to recombination and vibrational relaxation in the B state of ClF. A control pulse delayed by 30–100 ps leads to exclusive formation of the ionic bond Cl+F−. A switching contrast better than 1 to 10 can be achieved in both directions. The control scheme makes use of the time scale of vibrational relaxation. By femtosecond pump–probe spectroscopy we show the wave packet oscillations from the recombination dynamics in the cage lasting for 2 ps and determine the subsequent decay of vibrational energy.

Photothermal conversion dynamics in femtosecond and picosecond discrete laser etching of Cu-phthalocyanine amorphous film analysed by ultrafast UV–VIS absorption spectroscopy

Novel etching of Cu-phthalocyanine (CuPc) amorphous film which is characteristic of ultrashort laser irradiation was successfully confirmed by tuning Ti:Sapphire laser (780 nm) to 150 fs, 250 ps, or 100 ns, and the primary processes were investigated by fs pump-fs probe and ps pump-fs probe spectroscopic measurements. In the fs and ps laser ablation, we have found discrete laser etching in that the etch depth becomes constant and is independent of laser fluence above the ablation threshold, although gradual (normal) etching, in which the etch depth increase continuously with the fluence above ablation threshold, was observed in the ns laser ablation. The transient absorption spectral measurements reveal the nonlinear photothermal conversion processes, corresponding to exciton–exciton annihilation and cyclic multiphotonic absorption. Their time evolutions during and after the excitation pulse duration were considered and elucidated to depend strongly on the excitation pulse width. On the basis of these results, we discuss an ablation mechanism for the ps and fs ablation that the temperature elevation bringing about transient high pressure is responsible for discrete etching.

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Concept and numerical investigation of an ultrafast high-contrast high-power KrF laser system

The concept of an ‘all-excimer’ ultrafast high-power UV laser system implementing the KrF excimer medium both in the oscillator and in the amplifiers is presented. The system includes a short-pulse (10-11–10-10s) KrF oscillator employing fast pulse period Q-switching or truncated Brillouin scattering and a multipass KrF amplifier with a multilayer saturable absorber applied as a highly effective pulse compressor. By means of computer simulations, it is shown that the system, incorporating currently available KrF discharge modules and acridine dye as the saturable absorber, is capable of generating high-contrast pico-or subpicosecond pulses of near terawatt power level.

Ultrafast ultraviolet pump–probe apparatus

An experimental method based on subpicosecond pulse amplification in excimer gain modules for simultaneously generating both ultrafast, energetic UV excitation pulses and ultrafast UV probe continua is described. Subpicosecond pulses at both 308 and 248 nm, with energies of several millijoules at each wavelength, are available from this apparatus for purposes of excitation. A simultaneously generated probe continuum spans the range ~450 to ~235 nm. Both the probe continuum and the seed pulses for amplification at 248 nm are produced by focusing 308-nm pulses into high-pressure gases (Ar and H2, respectively). A deduced multiple-pulse structure in the probe continuum limits the time resolution of the apparatus to ~1 psec.

Ultrafast deep UV Lithography with excimer lasers

The use of high-power pulsed excimer lasers for photolithography is described for the first time. Short exposure times, high resolution and absence of speckle are experimentally demonstrated. Using a XeCl laser at 308 nm and a KrF laser at 248 nm, excellent quality images are obtained by contact printing in two positive photoresists. Resolution down to 1000 line-pairs/mm is demonstrated. These images are comparable to state-of-the-art lithography done with conventional lamps; the major difference is that the excimer laser technique is ∼ 2 orders of magnitude faster. Preliminary results on reciprocity behavior in several resists are also presented.