Relative Phase Change of Nearby Resonances in Temporally Delayed Sum Frequency Spectra.
Surface-selective sum frequency generation (SFG) spectroscopy has been previously shown to benefit from a finite time delay between two input laser pulses, which suppresses the nonresonant background and improves spectral resolution. Here we demonstrate another consequence of the time delay in SFG: depending on the magnitude of the delay, nearby resonances (e.g., vibrational modes) can "flip" their relative phase, i.e., appear either in-phase or out-of-phase with one another, resulting in either constructive or destructive interference in SFG spectra. This is significant for interpretation of the SFG spectra, in particular because the sign of the resonant amplitude provides the absolute molecular orientation (up vs down) of the vibrational chromophore. We present results and model calculations for symmetric and asymmetric CH-stretch modes of the methyl-terminated Si(111) surface, showing that the phase flip occurs when the delay matches half-cycle of the difference frequency between the two modes.
Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States.
Fadel Y
FY
Shalhout
Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States.
Sergey
S
Malyk
Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States.
Alexander V
AV
Benderskii
SFG line shapes; nonresonant background suppression; quantum beats; sum frequency generation; surface-selective nonlinear spectroscopy; vibrational coherences