Two-dimensional infrared spectroscopy

Nonlinear two-dimensional infrared spectroscopy (2DIR) is a technique that has the ability to correlate vibrational modes in condensed-phase systems. This technique provides information beyond linear infrared spectra, by spreading the vibrational information along multiple axes, yielding a frequency correlation spectrum. [cite journal | author = P. Hamm, M. H. Lim, R. M. Hochstrasser | title = Structure of the amide I band of peptides measured by femtosecond nonlinear-infrared spectroscopy
journal = J. Phys. Chem. B | volume = 102 | pages = 6123 | year = 1998 | doi = 10.1021/jp9813286] A frequency correlation spectrum can offer information structural information such as vibrational mode coupling, anharmonicities, along with chemical dynamics such as energy transfer rates and molecular dynamics with femtosecond time resolution. 2DIR experiments have only become possible with the development of ultrafast lasers and the ability to generate femtosecond infrared pulses. Among the many systems studied with infrared spectroscopy are, metal carbonyls|, short polypeptides, proteins, and DNA oligomers. [cite journal | author = S. Mukamel | title = Multidimensional Fentosecond Correlation Spectroscopies of Electronic and Vibrational Excitations | journal = Annual Review of Physics and Chemistry | volume =51 | pages = 691 | year = 2000 | doi = 10.1146/annurev.physchem.51.1.691 ] [cite journal | author = M. H. Cho | title = Coherent Two-Dimensional Optical Spectroscopy | journal = Chemical Reviews | volume =108 | pages = 1331-1418 | year = 2008 | doi = Coherent Two-Dimensional Optical Spectroscopy ]

There are two main approaches to two-dimensional spectroscopy, the Fourier-transform method, in which the data is collected in the time-domain and then Fourier-transformed to obtain a frequency-frequency 2D correlation spectrum, and the frequency domain approach in which all the data is collected directly in the frequency domain. The time-domain approach consists of applying two pump pulses. The first pulse creates a coherence between the vibrational modes of the molecule and the second pulse creates a population, effectively storing information in the molecules. After a determined waiting time, ranging from a zero to a few hundred picoseconds, an interaction with a third pulse again creates a coherence, which, due to an oscillating dipole, radiates an infrared signal. The radiated signal is heterodyned with a reference pulse in order to retrieve frequency and phase information; the signal is usually collected in the frequency domain using a spectrometer yielding detection frequency $omega\_3$. A two-dimensional Fourier-transform along $omega\_1$ then yields a ($omega\_1$, $omega\_3$) correlation spectrum.

Similarly, in the frequency-domain approach, a narrowband pump pulse is applied and, after a certain waiting time, the a broadband pulse probes the system. A 2DIR corrlation spectrum is obatained by plotting the probe frequency spectrum at each pump frequency.