Ph.D. in Chemistry, 2004, University of California at Berkeley

B.A. in Chemistry, 1995, Wesleyan University, Middletown, CT

We are currently developing the technique of femtosecond Raman gain spectroscopy (FRGS) in order to provide a means of obtaining vibrational spectra on the femtosecond time scale. In the past, the Fourier relationship between laser pulse duration and pulse bandwidth has hindered the development of time-resolved vibrational spectroscopies on the sub-100 fs time scale. A laser pulse 50 fs in duration will have a minimum bandwidth of 300 cm^-1, which can severely limit the frequency resolution possible in ultrafast experiments. A spontaneous Raman spectrum observed with such a pulse would show vibrational linewidths > 300 cm^-1, making any detailed structural analysis of the spectrum impossible. FRGS separates the time-resolution limit from the frequency resolution limit and thereby allows direct structural information to be determined from vibrational spectra taken as chemical reactions proceed on the ultrafast time-scale. FRGS uses three laser pulses to initiate a chemical reaction and then probe the system's vibrational spectrum. The first pulse incident on the sample is the ~50 fs actinic pump, which excites the molecule from the ground to the excited state and thereby initiates the photochemical reaction. The probe consists of the Raman pump, a narrow bandwidth pulse that is ~1 ps in duration, and the Raman probe, an ultrafast (~50 fs) duration pulse containing a continuum of frequencies to the red of the Raman pump frequency. The Raman pump¹s narrow bandwidth determines the frequency resolution of the resulting vibrational spectrum and its wavelength is chosen to be non-resonant with the chemical system, so it induces no chemical change in the system until the Raman probe arrives to receive the stimulated Raman signal. This makes the Raman pump's long time duration irrelevant to the time resolution of the experiment. The Raman probe experiences amplification at the specific Raman frequencies of the chemical system and this amplification can be detected by dispersing the pulse in a spectrometer and observing its spectrum in both the presence and absence of the Raman pump. The time resolution of the experiment is determined by the pulse-widths of the actinic pump and the Raman probe and therefore is on the order of 50 fs. Initially, we will use this technique to observe structural changes that drive the ultrafast relaxation dynamics of beta-carotene[McCamant, 2002]. Later, we will use FRGS to observe the structural changes that take place during the ultrafast primary event in vision, thereby building on our recent picosecond time-resolved results [Kim, 2001].

FRGS is a three-pulse technique:

Pulse 1 is the "actinic pump." This pulse excites the ground-state molecules and initiates the photochemical reaction. The actinic pump pulses from our non-collinear phase-matched OPA are ~20 fs long.

Pulse 2 is the "Raman pump." This is a long duration (~1 ps) pulse with a narrow frequency bandwidth. It is not resonant with any electronic transition of the molecule.

Pulse 3 is the "Raman probe." This is a ~50-80 fs pulse that contains a continuum of wavelengths to the red of the Raman pump. It experiences gain at the Stokes frequencies shifted off of the Raman pump wavelength.

• Comparison of the Raman Probe spectrum with and without the Raman Pump on generates a Raman Spectrum of the sample.
• Ultrafast time-resolution is obtained by delaying the Raman probe relative to the actinic pump.

References & Publications:

Kim, J.E.; McCamant, D.W.; Zhu, L.; Mathies, R.A. "Resonance Raman Structural Evidence that the Cis-to-Trans Isomerization in Rhodopsin Occurs in Femtoseconds" J. Chem. Phys. B v105:1240-1249 (2001):
[Abstract] [Full Text in HTML] [Full Text in PDF].

McCamant, D.W., Kim, J.E., Mathies, R.A. "Vibrational Relaxation in Beta-Carotene Probed by Picosecond Stokes and Anti-Stokes Resonance Raman Spectroscopy" J. Chem. Phys. A v106 n25: 6030-6038 (2002):
[Abstract] [Full Text in HTML] [Full Text in PDF].

McCamant, D. W., Kukura, P., and Mathies, R. A. Femtosecond Time-Resolved Stimulated Raman Spectroscopy: Applications to the Ultrafast Internal Conversion in b-Carotene, J. Phys. Chem., submitted (2003).