Picosecond Time-resolved Raman Spectra Research Articles

Picosecond near-infrared excited transient Raman spectra of β-carotene in the excited S 2 state: Solvent effects on the in-phase C [dbnd]C stretching band and vibronic coupling

Picosecond time-resolved Raman spectra of β-carotene in the excited S 2 ( 1 B u + ) state have been obtained in resonance with its near-infrared transient absorption. The Raman band due to the in-phase C C stretching mode of β-carotene in the S 2 state has been observed at about 1550 cm −1. The solvent effects on this band are discussed in comparison with the previous results on the corresponding bands in the ground S 0 ( 1 A g - ) state and the excited S 1 ( 2 A g - ) state.

Picosecond Time-Resolved Resonance Raman Study of the Solvated Electron in Water

Picosecond time-resolved Raman spectra of water were measured under the resonance condition with the electronic transition of the solvated electron. Transient Raman bands were observed in the OH bend and the OH stretch regions in accordance with the generation of the solvated electron. The lifetimes of the transient Raman bands were shortened by the addition of the electron scavenger, in exactly the same manner as the solvated electron absorption. It was concluded that the observed transient Raman bands are attributed to the water molecules that directly interact with the electron in the first solvation shell. The resonance enhancement factors were estimated as high as ∼105 (the OH bend) and ∼103 (the OH stretch) when the probe wavelength was tuned to the absorption maximum of the s → p transition of the solvated electron. The observed very high resonance enhancement indicated that the vibrational state of the solvating water molecules is strongly coupled with the electronic state of the electron. This im...

Analysis of the Solvent- and Temperature-Dependent Raman Spectral Changes of S1trans-Stilbene and the Mechanism of the trans to cis Isomerization: Dynamic Polarization Model of Vibrational Dephasing and the CC Double-Bond Rotation

Solvent- and temperature-dependent band shape changes of the olefinic CC stretch Raman band of S1 trans-stilbene have been analyzed on the basis of the dynamic polarization model. The analysis has shown that the solvent-induced dynamic polarization gives rise to the dephasing of the CC stretch vibration and, concomitantly, triggers and facilitates the rotation around the CC bond leading to the trans to perpendicular (and eventually to cis) isomerization. Picosecond time-resolved Raman spectra have been measured in the three alkane solvents, hexane, octane, and decane at a number of different temperatures ranging from 268 to 338 K and a total of 40 peak positions and the bandwidths have been determined for the CC stretch band. The correlation plot of the bandwidth against peak position shows a clear linear relationship that is predicted by the dynamic polarization model. Picosecond time-resolved fluorescence measurements have been performed in the same three alkane solvents at five different temperatures f...

Novel Resonance Raman Enhancement of Local Structure around Solvated Electrons in Water

Picosecond time-resolved Raman spectra of water were measured under the resonance condition with the s → p electronic transition of the solvated electron for the first time. We found that a strong transient Raman band appears in the frequency region 30 cm-1 lower than the OH-bend band of bulk water in accordance with the formation of solvated electrons. The observed transient Raman band was assigned to the vibration of the solvating water molecules that strongly interact with the solvated electron in the first solvation shell. The mechanism of this novel resonance enhancement is discussed on the basis of the vibronic theory.

Time-resolved resonance Raman spectra of the intramolecular charge transfer state of DMABN

Picosecond time-resolved Raman spectra of the intramolecular charge transfer state of DMABN and DMABN-d 6, have been obtained in resonance with two different electronic states, using probes at 330 and 400 nm. Fluorescence was rejected using a Kerr gate. Several resonance Raman bands in the 750–2200 cm −1 region are reported for the first time. The prominent CN stretch at 2096 cm −1 and bands sensitive to the amino group deuteration are only observed using the 330 nm probe. At 400 nm, bands assigned to modes localised on the ring dominate the spectrum. The results support the TICT model.

Picosecond Time-Resolved Raman Study of trans-Azobenzene

The electronic and vibrational relaxation of photoexcited trans-azobenzene was investigated in solution by picosecond time-resolved Raman spectroscopy. Picosecond time-resolved Raman spectra were measured with the probe wavelength at 410 nm, which is in resonance with a transient absorption appearing after the S2(ππ*) ← S0 photoexcitation. Several transient Raman bands assignable to the S1 state were observed immediately after photoexcitation. The lifetime of the S1 state showed a significant solvent dependence, and it was determined as ∼12.5 ps in ethylene glycol and ∼1 ps in hexane. Time-resolved anti-Stokes Raman measurements were also carried out for hexane solutions to obtain information about vibrational relaxation process. The anti-Stokes spectra showed that the observed S1 state was highly vibrationally excited. In addition, several anti-Stokes Raman bands due to the S0 state were observed after the decay of the S1 state, indicating that the vibrationally excited S0 state was generated after elect...

Femtosecond–picosecond dynamics of the lowest excited singlet state in α-quinquethiophene

A femtosecond photo-induced absorption spectrum has been studied for α-quinquethiophene. The observed stimulated emission with a lifetime of 600±40 fs is ascribed to the transition from the geometrically non-relaxed levels in the lowest excited singlet (S 1) state. A picosecond time-resolved Raman spectrum studied by utilizing a time-frequency two-dimensional multiplex CARS spectrometer has provided signals due to the geometrically non-relaxed S 1 state as well as the relaxed S 1 state.

Photoinduced Solvent Ligation to Nickel(II) Octaethylporphyrin Probed by Picosecond Time-Resolved Resonance Raman Spectroscopy

Pump/probe picosecond time-resolved resonance Raman spectra of nickel(II) octaethylporphyrin (NiOEP) in pyridine were observed in the time range −5 to +1000 ps. The spectra demonstrate generation of the vibrationally and electronically excited (d,d) state (B1g) immediately after the excitation to the ππ* state of the macrocycle, subsequent vibrational relaxation, and the formation of six-coordinate exciplex, B1g(L)2 (L = pyridine). Singular value decomposition analysis was applied to a series of picosecond time-resolved Raman spectra to investigate whether a five-coordinate complex was generated prior to the formation of the six-coordinate species. The results indicate insignificant population of the five-coordinate species preceding the formation of B1g(L)2 and suggest concerted coordination of two solvent molecules to axial positions of the (d,d) excited NiOEP. The formation process of the B1g(L)2 species is discussed.

Picosecond structural relaxation of S 1 trans-stilbene in solution as revealed by time-resolved Raman spectroscopy

Picosecond time-resolved Raman spectra of S 1 trans-stilbene (S 1 tSB) in heptane have been obtained with transform-limited (3 ps and 2 cm −1) picosecond pulses from a high repetition-rate amplified synchronously-pumped dye laser system. Both the peak positions and the bandwidths of two olefinic bands, the CC stretch around 1565 cm −1 and the CPh stretch around 1180 cm −1, change with time. The changes in the bandwidths precede those of the peak positions and start immediately after the photoexcitation. The peak positions remain unaltered during the first few picoseconds of the photoexcitation and then start to change. This suggests that the changes of the peak positions are not solely due to an intramolecular process but reflect certain structural relaxation of S 1 tSB that involves the motion of the surrounding solvent molecules.

Picosecond time-resolved resonance Raman studies of hemoglobin: implications for reactivity.

Picosecond time-resolved Raman spectra of hemoglobin generated with blue pulses (20 to 30 picoseconds) that were resonant with the Soret band and of sufficient intensity to completely photodissociate the starting liganded sample are reported. For both R- and T-state liganded hemoglobins, the peak frequencies in the spectrum of the deoxy transient were the same at approximately 25 picoseconds as those observed at 10 nanoseconds subsequent to photodissociation. In particular, the large R-T differences in the frequency of the stretching mode for the iron-proximal histidine bond (VFe-His) detected in previously reported nanosecond-resolved spectra were also evident in the picosecond-resolved spectra. The implications of this finding with respect to the distribution of strain energy in the liganded protein and the origin of the time course for geminate recombination are discussed. On the basis of these results, a microscopic model is proposed in which delocalization of strain energy is strongly coupled to the coordinate of the iron. The model is used to explain the origin of the R-T differences in the rates of ligand dissociation.