Heterodyning of Interface-Selective Nonlinear Spectroscopy
Tahei TAHARA
Molecular Spectroscopy Laboratory, Advanced Science Institute (ASI), RIKEN

One of the most important targets of recent molecular science is soft interfaces such asliquid interfaces. We recently developed new interface-selective electronic and vibration
nonlinear spectroscopies to study soft interfaces [1-6]. Among them, multiplex electronic
sum-frequency generation (ESFG) spectroscopy and femtosecond time-resolved ESFG
(TR-ESFG) spectroscopy enabled us to obtain steady-state and time-resolved electronic
spectra at liquid interfaces with unprecedented high quality. These new interface-selective
electronic spectroscopies attract much attention as new powerful tools for study of softinterfaces [7]. ESFG spectroscopy, however, does not provide information of χ (2) itself but
gives data about its square modulus, i.e., |χ (2) | 2 , because the intensity of the nonlinear opticalsignal is directly detected (homodyne detection).
This drawback of the homodyne ESFG spectroscopy can be overcome by heterodyne
detection in which the signal light is mixed with local oscillator with a controlled phaserelation. We extended the original ESFG spectroscopy to heterodyne measurements byutilizing a tandem configuration, as shown in Fig. 1. In this configuration, a liquid sample tobe studied is set at one sample point, and GaAs, which generates LO, is placed at the other.
The interference of the SFG signals from the sample and GaAs is detected with a spectrographand a CCD. We successfully achieved heterodyne detection of ESFG for the first time(HD-ESFG) [8]. On the basis of this HD-ESFG scheme, we also carried out heterodyning ofvibrational sum-frequency generation (VSFG). We obtained the imaginary part and real part ofFig. 1 Setup of heterodyne detected electronic sum-frequency generation (HD-ESFG)
spectroscopy

Fig. 2 HD-ESFG spectra of p-nitroaniline (PNA: left) and N,N’-diethyl-p-nitoroaniline(DEPNA: right). at the air/fused-silica interface (red: imaginary part; black: real part). The
opposite sign of the spectra manifest the opposite orientation of PNA and DEPNA at theinterface.

vibrationally resonant χ (2) spectra in multiplex VSFG measurements for the first time [9].
With the heterodyning of ESFG and VSFG, we are now able to directly obtain
experimental data that are linearized to electronically resonant or vibrationally resonant χ (2) .
One of the unique information obtainable with the heterodyned second-order nonlinear
spectroscopy is the absolute orientation of interfacial molecules. Actually, we observed
flip-flop of p-nitroaniline derivatives at the air/glass interface with HD-ESFG (Fig. 2) [8] andrevealed flip-flop of water molecules at the air/water interface with HD-VSFG [9].

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9.

S. Nihonyanagi, S. Yamaguchi and T. Tahara, submitted.