TY - JOUR
T1 - Taking apart the two-dimensional infrared vibrational echo spectra
T2 - More information and elimination of distortions
AU - Kwak, Kyungwon
AU - Rosenfeld, Daniel E.
AU - Fayer, M. D.
N1 - Funding Information:
The authors would like to thank the Air Force Office of Scientific Research (F49620-01-1-0018) for support of this work. Daniel E. Rosenfeld was supported by the Fannie and John Hertz Foundation and a Stanford Graduate Fellowship.
PY - 2008
Y1 - 2008
N2 - Ultrafast two-dimensional infrared (2D-IR) vibrational echo spectroscopy can probe the fast structural evolution of molecular systems under thermal equilibrium conditions. Structural dynamics are tracked by observing the time evolution of the 2D-IR spectrum, which is caused by frequency fluctuations of vibrational mode(s) excited during the experiment. However, there are a variety of effects that can produce line shape distortions and prevent the correct determination of the frequency-frequency correlation function (FFCF), which describes the frequency fluctuations and connects the experimental observables to a molecular level depiction of dynamics. In addition, it can be useful to analyze different parts of the 2D spectrum to determine if dynamics are different for subensembles of molecules that have different initial absorption frequencies in the inhomogeneously broadened absorption line. Here, an important extension to a theoretical method for extraction of the FFCF from 2D-IR spectra is described. The experimental observable is the center line slope (CLS ωm) of the 2D-IR spectrum. The CLS ωm is obtained by taking slices through the 2D spectrum parallel to the detection frequency axis (ωm). Each slice is a spectrum. The slope of the line connecting the frequencies of the maxima of the sliced spectra is the CLS ωm. The change in slope of the CLS ωm as a function of time is directly related to the FFCF and can be used to obtain the complete FFCF. CLS ωm is immune to line shape distortions caused by destructive interference between bands arising from vibrational echo emission, from the 0-1 vibrational transition (positive), and from the 1-2 vibrational transition (negative) in the 2D-IR spectrum. The immunity to the destructive interference enables the CLS ωm method to compare different parts of the bands as well as comparing the 0-1 and 1-2 bands. Also, line shape distortions caused by solvent background absorption and finite pulse durations do not affect the determination of the FFCF with the CLS ωm method. The CLS ωm can also provide information on the cross correlation between frequency fluctuations of the 0-1 and 1-2 vibrational transitions.
AB - Ultrafast two-dimensional infrared (2D-IR) vibrational echo spectroscopy can probe the fast structural evolution of molecular systems under thermal equilibrium conditions. Structural dynamics are tracked by observing the time evolution of the 2D-IR spectrum, which is caused by frequency fluctuations of vibrational mode(s) excited during the experiment. However, there are a variety of effects that can produce line shape distortions and prevent the correct determination of the frequency-frequency correlation function (FFCF), which describes the frequency fluctuations and connects the experimental observables to a molecular level depiction of dynamics. In addition, it can be useful to analyze different parts of the 2D spectrum to determine if dynamics are different for subensembles of molecules that have different initial absorption frequencies in the inhomogeneously broadened absorption line. Here, an important extension to a theoretical method for extraction of the FFCF from 2D-IR spectra is described. The experimental observable is the center line slope (CLS ωm) of the 2D-IR spectrum. The CLS ωm is obtained by taking slices through the 2D spectrum parallel to the detection frequency axis (ωm). Each slice is a spectrum. The slope of the line connecting the frequencies of the maxima of the sliced spectra is the CLS ωm. The change in slope of the CLS ωm as a function of time is directly related to the FFCF and can be used to obtain the complete FFCF. CLS ωm is immune to line shape distortions caused by destructive interference between bands arising from vibrational echo emission, from the 0-1 vibrational transition (positive), and from the 1-2 vibrational transition (negative) in the 2D-IR spectrum. The immunity to the destructive interference enables the CLS ωm method to compare different parts of the bands as well as comparing the 0-1 and 1-2 bands. Also, line shape distortions caused by solvent background absorption and finite pulse durations do not affect the determination of the FFCF with the CLS ωm method. The CLS ωm can also provide information on the cross correlation between frequency fluctuations of the 0-1 and 1-2 vibrational transitions.
UR - http://www.scopus.com/inward/record.url?scp=44449086941&partnerID=8YFLogxK
U2 - 10.1063/1.2927906
DO - 10.1063/1.2927906
M3 - Article
C2 - 18513030
AN - SCOPUS:44449086941
SN - 0021-9606
VL - 128
JO - Journal of Chemical Physics
JF - Journal of Chemical Physics
IS - 20
M1 - 204505
ER -