Dipak Kumar Das

Direct Observation of Coherent Oscillations in Solution due to Microheterogeneous Environment

We report, for the first time, direct observation of coherent oscillations in the ground-state of IR775 dye due to microheterogeneous environment. Using ultrafast near-infrared degenerate pump-probe technique centered at 800 nm, we present the dynamics of IR775 in a binary mixture of methanol and chloroform at ultra-short time resolution of 30 fs. The dynamics of the dye in binary mixtures, in a time-scale of a few fs to ~740 ps, strongly varies as a function of solvent composition (volume fraction). Multi-oscillation behavior of the coherent vibration was observed, which increased with decreasing percentage of methanol in the dye mixture. Maximum number of damped oscillations were observed in 20% methanol. The observed vibrational wavepacket motion in the ground-state is periodic in nature. We needed two cosine functions to fit the coherent oscillation data as two different solvents were used. Dynamics of the dye molecule in binary mixtures can be explained by wavepacket motion in the ground potential energy surface. More is the confinement of the dye molecule in binary mixtures, more is the number of damped oscillations. The vibrational cooling time, τ2, increases with increase in the confinement of the system. The observed wavepacket oscillations in ground-state dynamics continued until 1.6 ps.

Observing ground state vibrational coherence and excited state relaxation dynamics of a cyanine dye in pure solvents.

Using a degenerate pump probe technique at 800 nm, Ground State Vibrational Coherence (GSVC) of a cyanine dye (IR780) is explored in various solvents. Five representative solvents were studied, of which GSVC is present in three: methanol (MeOH), ethanol (EtOH) and dichloromethane (DCM). The maximum number of damped oscillations was observed in DCM solvent and the oscillations persisted for 1.2 ps. However, GSVC is absent in two solvents: acetonitrile (ACN) and dimethyl sulfoxide (DMSO). For nondegenerate pump probe studies with a 400 nm pump and a supercontinuum probe, GSVC was absent for IR780 in all of the five solvents studied. Instead these studies showed excited state absorption corresponding to S1 → Sn absorption bands in the 450 to 650 nm range. Decay profiles for these correspond to the higher electronic states (Sn, n ≥ 2) in contrast to the degenerate case involving S1. Complete dynamics of IR780 with 30 fs pulses are reported by invoking three time-constants to fit the kinetic traces that can, in turn, be assigned as: vibrational cooling, intermediate state time, and ground state recovery (GSR). All these dynamics exhibit noticeable solvent dependence. A plateau region is also observed in the early part of the GSR dynamics, strongly suggesting the existence of an intermediate state between the ground (S0) and excited singlet (S1) states.

Sensing near the liquid:liquid interface remotely via ultrafast pump probe study

Ultrafast pump-probe technique has been invoked to understand the liquid:liquid interface by using a cyanine dye dissolved in dimethyl sulfoxide (DMSO), interfaced with neat diethyl-ether (Et2O). We show that the dynamics of the dye gets slow down on moving from the bulk dye solution in DMSO towards the interface with neat-diethyl ether. Hence we get an alternate picture of interface by looking at the time constants. Four time constants were used for fitting the experimental data which are assigned to be coherent artifact, vibrational cooling, intermediate state time and finally ground state recovery time. The drastic change in signal was observed at a distance of 0.1 mm from the interface. The decreasing time constants from bulk to near interfaced solution strongly suggests involvement of heterogeneity in the system because of the penetration of diethyl ether through the interface into the DMSO layer where the dye is dissolved, which strongly affects the dynamics at near the interface of the DMSO with neat-diethyl ether layer (at 0.1 mm distance from the interface).

Control of femtosecond laser driven retro-Diels-Alder-like reaction of dicyclopentadiene

Using femtosecond time resolved degenerate pump-probe mass spectrometry coupled with simple linearly chirped frequency modulated pulse, we elucidate that the dynamics of retro-Diels-Alder reaction of diclopentadiene (DCPD) to cyclopentadiene (CPD) in supersonic molecular beam occurs in ultrafast time scale. Negatively chirped pulse enhances the ion yield of CPD, as compared to positively chirped pulse. This indicates that by changing the frequency (chirp) of the laser pulse we can control the ion yield of a chemical reaction.