Alexander N. Tarnovsky

Photodissociation of aryl halides in the gas phase studied with femtosecond pump-probe spectroscopy

Abstract The photodissociation dynamics of iodo-, bromo- and chlorobenzene upon excitation at 266 nm have been studied by femtosecond pump-probe spectroscopy combined with time-of-flight (TOF) mass spectrometric detection. The kinetics are characterized by a single time constant for chlorobenzene (1 ns) and bromobenzene (28 ps), and by two time constants for iodobenzene (700 and 350 fs). The time constant of 350 fs is due to direct dissociation. The increase of the other time constants with decreasing halogen mass supports the assignment of these constants to the decay of the initially excited ( π , π ∗ ) states to the repulsive triplet ( n , σ ∗ ) state due to spin–orbit coupling.

Photodissociation dynamics of diiodomethane in solution

Abstract We have studied the photodissociation dynamics of diiodomethane (CH2I2) in acetonitrile by femtosecond pump–probe spectroscopy. The reaction was initiated using 310 nm light and the transient absorption was measured over a spectral region from 290 to 1220 nm. The wavepacket leaves the initially excited Franck–Condon region on the CH2I2 potential surface on a timescale of 350 fs. The CH2I fragment is observed within the timespan of 0.3–3 ps. The hot photoproduct, assigned to be the isomer of diiodomethane CH2I–I, appears with a characteristic time of 1 ps and vibrationally relaxes on a timescale of 10 ps.

On the use of two-photon absorption for determination of femtosecond pump-probe cross-correlation functions

Abstract We have investigated the applicability of two-photon absorption signals ( hν pump + hν probe ) in common solvents for determination of the pump–probe cross-correlation function in transient absorption experiments. Ten different solvents, including water, lower alcohols, n -hexane, n -heptane, acetonitrile, dichloromethane, chloroform and tetrachloromethane, were studied with UV excitation and UV-visible probe pulses. We report on how the signals are influenced by the relative polarization of the pump and probe beams, the excitation intensity and the detection bandwidth. Other contributions to the signals are also discussed.

Photodissociation of bromobenzene in solution

The photodissociation of bromobenzene in solution was investigated with ultrafast transient absorption spectroscopy following excitation at 266 nm. Ab initio calculations of lower singlet and triplet states were performed in order to guide the interpretations. The main feature of the kinetics measured between 300 and 930 nm in acetonitrile is a 9±1 ps decay, which we mainly assign to predissociation. Similar decays were observed in hexane, dichloromethane and tetrachloromethane at 400 and 800 nm. Other features in acetonitrile, such as complicated short-time dynamics between 420 and 620 nm and a long-lived component, might indicate the involvement of lower triplet states.

Ultrafast time‐resolved X‐ray absorption spectroscopy of chemical systems

X-ray absorption spectroscopy (XAS) is a well-established technique to probe the electronic and structural properties of a large class of systems, ranging from solid-state materials to proteins [1]. X-ray spectra are characterized by a series of edge features, which arise due to the extraction of a core electron from the inner K, L, M, etc. shells to the ionization limit. Zooming into one of the edges reveals a variety of fine structure near the edge, the so-called XANES or X-ray absorption near edge structure, while at higher energies above the edge the so-called EXAFS or extended X-ray absorption fine structure, can be observed. In many-body systems, these features are caused by the presence of neighboring atoms and lie in the fact that the photoelectron scatters off the neighboring atoms and the backscattered wave interferes with the outgoing wave. This interference gives rise to weak modulations of the otherwise smoothly varying absorption cross-section as a function of Xray energy. The proper analysis of these features delivers information about the local geometric structure to high accuracy. For studying chemical properties, XAS has the following advantages:

Impurity-Localized Electronic Changes in Eu:SrGa_2S_4 Studied by Ultrafast X-Ray Absorption Spectroscopy

The time-dependent electronic structure of Eu:SrGa2S4 after excitation by a femtosecond laser pulse is investigated by time-resolved x-ray absorption spectroscopy. We observe a transient shift in the Eu L3 edge consistent with Eu ionization.

Structural dynamics and electronic structure changes probed with lasers and X-rays

Several new results and opportunities for ultrafast x-ray absorption spectroscopy (XAS) are demonstrated. It is possible to work with highly dil. solns. in transmission mode without dramatic loss of signal-to-noise ratio. This is very promising as one can envision the study of samples, for which large concns. are impossible to reach. Moreover, it is possible to scan the time delay between the laser pump pulse and the x-ray probe pulse, and therefore follow the evolution of the system from the start. The operation of an optical x-ray cross-correlator is also demonstrated. The time resoln. is not a limiting factor and the expts. are feasible with sources of shorter x-ray pulses, provided the flux is not too low. [on SciFinder (R)]

Photodissociation dynamics of dihalomethanes in solution studied by ultrafast pump-probe spectroscopy

Summary form only given. Much effort has been put into understanding the photodissociation dynamics of dihalomethanes in the gas phase. In solution, the task is even more challenging since supplementary processes can be present in the condensed phase; one such example is geminate recombination within the cage of solvent molecules. We present observations of the isomers for dihalomethanes, with particular focus on chloroiodomethane (CH/sub 2/ICl). The photodissociation of CH/sub 2/ICI dissolved in acetonitrile was initiated using a femtosecond pulse at 266 nm and the transient absorption was measured out to delays of 300 ps over a spectral region from 330 to 850 nm. The photodissociation leads to formation and subsequent vibrational cooling of a product species, which we suggest is the isomer of chloroiodomethane, CH/sub 2/Cl-I, likely produced via in-cage recombination between the nascent fragments CH/sub 2/Cl and I.