Multi-mode heterodyned 5th-order infrared spectroscopy.

Abstract

Fifth-order multidimensional infrared spectroscopy with heterodyned detection was carried out in the three-beam dual-frequency configuration. Numerous 5th-order cross peaks were detected for the 4-azidobutyrate-N-hydroxysuccinimide ester compound in solution involving several vibrational modes ranging in frequency from 1045 to 2100 cm-1. Cross peaks involving overtones (2X/Z) and combination bands (XY/Z) among the tags, modes X and Y excited by the first two mid-IR laser pulses, and the reporter, modes Z excited by the third laser pulse, were acquired and the factors affecting the amplitude of 5th-order cross peaks are discussed. The 5th-order cross peaks were detected among modes that are spatially close (a few bonds apart) as well as for modes spatially separated by ca. 12 Å (eight bonds apart). In both cases, the waiting time dependences for the 3rd and 5th order cross peaks were found to be different. In particular, the waiting time at which the cross-peak maximum is reached, the decay time, and the value of a plateau at large waiting times were all differing strongly. The differences are explained by reduced sensitivity of the 5th-order signals to modes coupled weakly to the reporter mode and different relaxation dynamics involving overtone state of the tag. The ability of the 5th-order peaks to single out the modes coupled strongly to the reporter can help identifying specific energy relaxation and transport pathways, which will be useful for understanding energy transport dynamics in molecules. The absorptive 5th-order cross peaks were constructed which report on three-point correlation functions. It is shown that in addition to the triple-frequency correlation functions, a correlation of the frequencies with the mode coupling (anharmonicity) can be naturally measured by the 5th-order spectroscopy. The current limit for detecting 5th-order signals was estimated at the level of 1 × 10-3 in reduced anharmonicity, which is determined by the corresponding two-state anharmonicity divided by the reporter mode spectral width. Given the simplicity of recording the 5th-order cross peaks in the three-beam configuration, the approach carries a potential for a broad use.