Picosecond spectroscopy experimental results

All these features were observed experimentally for solutions of 3-amino-/V-methylphthalimide, 4-amino-/V-methylphthalimide, and for nonsubstituted rhoda-mine. The results were observed for cooled, polar solutions of phthalimides, in which the orientational relaxation is delayed. Exactly the same spectral behavior was observed [50] by picosecond spectroscopy for low viscosity liquid solutions at room temperature, in which the orientational relaxation rate is much higher. All experimental data indicate that correlation functions of spectral shifts Av-l(t), which are used frequently for describing the Time Dependent Stokes Shift, are essentially the functions of excitation frequency. 

When an electron is injected into a polar solvent such as water or alcohols, the electron is solvated and forms so-called the solvated electron. This solvated electron is considered the most basic anionic species in solutions and it has been extensively studied by variety of experimental and theoretical methods. Especially, the solvated electron in water (the hydrated electron) has been attracting much interest in wide fields because of its fundamental importance. It is well-known that the solvated electron in water exhibits a very broad absorption band peaked around 720 nm. This broad absorption is mainly attributed to the s- p transition of the electron in a solvent cavity. Recently, we measured picosecond time-resolved Raman scattering from water under the resonance condition with the s- p transition of the solvated electron, and found that strong transient Raman bands appeared in accordance with the generation of the solvated electron [1]. It was concluded that the observed transient Raman scattering was due to the water molecules that directly interact with the electron in the first solvation shell. Similar results were also obtained by a nanosecond Raman study [2]. This finding implies that we are now able to study the solvated electron by using vibrational spectroscopy. In this paper, we describe new information about the ultrafast dynamics of the solvated electron in water, which are obtained by time-resolved resonance Raman spectroscopy. 

In addition to ab initio quantum simulations of the experimental femtosecond and picosecond pump-probe spectra, traditional continuous-wave (CW) spectra could also be simulated using the time-dependent approach to absorption spectroscopy [13]. The results show that femtosecond/picosecond versus CW spectroscopy is complementary in the present case, the radial and angular pseudorotation and the symmetric stretch are observed, and simulated with preferential sensitivity using CW picosecond, and femtosecond spectroscopy, respectively. 

We have performed picosecond time resolved absorption spectroscopy for organic dyes in alcoholic solution and have shown the following results. The recXral shape of the difference spectrum before and after the excitation is expressed as the superposition of the absorption and fluorescence spectra detected under steady state condition when the solvent relaxation time is sufficiently short compared with the time resolution of the experimental equipment and the excited state lifetime. On the other hand, the spectrum in the viscous solvent at low temperature shows slightly sharp in initial and broadens its shape with time. 

In the weakly anharmonic molecular crystal the natural modes of vibration are collective, with each internal vibrational state of the molecules forming a band of elementary excitations called vibrons, in order to distinguish them from low-frequency lattice vibrations known as phonons. Unlike isolated impurities in matrices, vibrons may be studied by Raman spectroscopy, which has lead to the establishment of a large body of data. We will briefly attempt to summarize some of the salient experimental and theoretical results as an introduction to some new developments in this field, which have mainly been incited by picosecond coherent techniques. 

Lifetime measurements were carried out on TPE in various solvents utilizing picosecond absorption spectroscopy [59,60]. Two absorption bands, 430 nm and 630 nm, were found for TPE excitation. The absorption band at 630 nm was found to be very weak and the decay was very rapid, within the experimental conditions. The absorption band around 430 nm was taken up for detail studies and its decay was studied in various solvents [61]. The solvent polarity is found to have a profound role in the decay of the 430 nm absorption band of excited TPE [61] and furthermore a linear relationship between log k (decay rate constant) and solvent polarity parameter was established. Based on these results, the authors mentioned that the 430-nm absorption band of singlet excited TPE has the twisted zwitterionic structure (as depicted in 1) and is nonfluorescent [61]. 

Whereas CM in bulk materials is usually determined in photocurrent device measurements, that is, by collecting the carriers, CM in QDs is studied by (optical) spectroscopic measurements, in which the orbital occupation of the QDs is probed on ultrafast (picosecond) timescales. Hence, the commonly used experimental procedures to determine CM in QDs (ultrafast spectroscopy) and in bulk (device measurements) are rather different. While time-resolved optical and IR spectroscopies are ideally suited to probe carrier populations in colloidal QDs, " light of terahertz (THz) frequencies interacts strongly with free carriers in the bulk material and allows the direct characterization of carrier density and mobility. From THz-time domain spectroscopy (TDS) experiments, one can quantitatively assess the number of photogenerated carriers in bulk semiconductors picoseconds after the light is absorbed. Additionally, as a result of the contact-free nature of the THz probe, it is possible to determine the CM factor in isolated samples of bulk semiconductors without the need to apply contacts, which is necessary in the device measurements. For these reasons, THz-TDS experiments have been employed to quantify CM in bulk PbSe and PbS on ultrafast timescales " in order to make a bulk-QD comparison in the context of the CM controversy. The CM factor in bulk PbS and PbSe was determined for excitation with various photon energies from the UV to the IR. 

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