Comparisons of 2D IR measured spectral diffusion in rotating frames using pulse shaping and in the stationary frame using the standard method.

Multidimensional visible spectroscopy using pulse shaping to produce pulses with stable controllable phases and delays has emerged as an elegant tool to acquire electronic spectra faster and with greatly reduced instrumental and data processing errors. Recent migration of this approach using acousto-optic modulator (AOM) pulse shaping to the mid-infrared region has proved useful for acquiring two dimensional infrared (2D IR) vibrational echo spectra. The measurement of spectral diffusion in 2D IR experiments hinges on obtaining accurate 2D line shapes. To date, pulse shaping 2D IR has not been used to study the time-dependent spectral diffusion of a vibrational chromophore. Here we compare the spectral diffusion data obtained from a standard non-collinear 2D IR spectrometer using delay lines to the data obtained from an AOM pulse shaper based 2D IR spectrometer. The pulse shaping experiments are performed in stationary, partially rotating, and fully rotating reference frames and are the first in the infrared to produce 2D spectra collected in a fully rotating frame using a phase controlled pulse sequence. Rotating frame experiments provide a dramatic reduction in the number of time points that must be measured to obtain a 2D IR spectrum, with the fully rotating frame giving the greatest reduction. Experiments were conducted on the transition metal carbonyl complex tricarbonylchloro(1,10-phenanthroline)rhenium(I) in chloroform. The time dependent data obtained from the different techniques and with different reference frames are shown to be in agreement.

Control of laser induced molecular fragmentation of n-propyl benzene using chirped femtosecond laser pulses.

We present the effect of chirping a femtosecond laser pulse on the fragmentation of n-propyl benzene. An enhancement of an order of magnitude for the relative yields of C3H3+ and C5H5+ in the case of negatively chirped pulses and C6H5+ in the case of positively chirped pulses with respect to the transform-limited pulse indicates that in some fragmentation channel, coherence of the laser field plays an important role. For the relative yield of all other heavier fragment ions, resulting from the interaction of the intense laser field with the molecule, there is no such enhancement effect with the sign of chirp, within experimental errors. The importance of the laser phase is further reinforced through a direct comparison of the fragmentation results with the second harmonic of the chirped laser pulse with identical bandwidth.

Spectrally resolved photon echo spectroscopy of Zn(II), Co(II) and Ni(II)-octaethyl porphyrins.

Spectrally resolved femtosecond three-pulse photon echo signal from some metal-octaethyl porphyrins (OEPs) like Zn(II)-OEP, Ni(II)-OEP, Co(II)-OEP is reported. Excited state dynamics is studied by time evolving photon echo spectra for different values of coherence and population relaxation times. Dependence on the spectrally resolved photon echo spectra on varying metal center is analyzed. For all these metallo-porphyrins, the electronic relaxation timescale is found to be limited by our laser pulsewidth of 50 fs whereas the timescale for intramolecular vibrational relaxation, occurring within the Q(00) band was found to be over a picosecond for Co(II)-OEP and Ni(II)-OEP and within a picosecond for Zn(II)-OEP.