Classical- and Heterodyne-Detected Vibrational Sum …
95
As shown in Eq. 19, the SFG-intensity spectrum, recorded with classical-VSFG
spectrometer, contains not only the real and imaginary components of resonantχ
(2)
i jk , but also the non-resonant-χ
(2)
i jk which appears as constant background with
the Re
χ
(2)
i jk
signal. In the case of weaker resonant components (weak signal), the
relative contribution of χ
(2)
N R can be significant, which changes the line-shape of the
I SFG signal from that of the Im
χ
(2)
i jk
signal. This issue becomes further complicated
for multiple overlapping bands.
The SFG signal and the incident ω 1 and ω 2 can be categorized by the direction
of their electric fields with reference to the plane of incidence. Electric field vectors
that lie with the plane of incidence are ‘P-polarized’ and those perpendicular to the
same are known as ‘S-polarized’. S-polarized light contains electric field only along
y-axis; whereas, the P-polarized light contains two electric field components along
x and z axes, respectively (Fig. 1). Polarization of the SFG light entirely depends on
the non-zero χ
(2)
i jk ; selective measurement of a specific component of χ
(2)
i jk is possible
by proper choice of polarization combination of ω 1 , ω 2 and the SFG light. The SFGintensity spectra at different polarization combinations (SSP, SPS, PSS and PPP)
are used to deduce information about orientational/conformational distribution of
interfacial molecules [8].
3 Narrowband Classical-VSFG Spectrometer
Figure 4 shows the typical optical layout of a narrowband classical-VSFG spectrometer. It is called ‘narrowband’ and ‘classical’ to indicate that the IR pulse used for
VSFG has a narrow spectral width and the SFG signal is detected as the conventional intensity spectrum (I SFG ∝ |E SFG |
2 ), respectively. The output from a laser,
such as a mode-locked Nd:YAG laser (∼30 ps, 1064 nm, 50 Hz, 40 mJ/pulse), is
divided into two parts; the first part is frequency doubled to 532 nm with a secondharmonic generator. The 532 nm beam is again split into two; one is used as the
visible pulse (ω V I S ; ∼500 μJ/pulse) for SFG and the other is mixed with the second
part of the 1064 nm fundamental to produce a tuneable infrared pulse in an optical
parametric amplifier and difference frequency generator unit (OPA-DFG unit). The
narrowband IR pulse (ω I R ; ∼260 μJ/pulse @ 3300 cm
−1 ) is spatially and temporally overlapped with ω V I S on the sample surface (e.g., water surface) while the
IR frequency is tuned stepwise to cover the spectral region of a desired vibrational
band. The intensity of the SFG is detected by a single channel detector consist of
a monochromator and photomultiplier tube (PMT). It is obvious that for a vibrational transition in the mid-IR region, the corresponding SFG signal appears in the
visible region (ω I R
3400cm
−1
+ω V I S (532nm) = ω SF (450nm)), and hence, visible
detector such as PMT is used which is easier to handle than that of IR-detector. To
remove the influence of the energy profile of the input IR and visible light, the
measured spectra are normalized with the non-resonant SFG signal from the surface
95
As shown in Eq. 19, the SFG-intensity spectrum, recorded with classical-VSFG
spectrometer, contains not only the real and imaginary components of resonantχ
(2)
i jk , but also the non-resonant-χ
(2)
i jk which appears as constant background with
the Re
χ
(2)
i jk
signal. In the case of weaker resonant components (weak signal), the
relative contribution of χ
(2)
N R can be significant, which changes the line-shape of the
I SFG signal from that of the Im
χ
(2)
i jk
signal. This issue becomes further complicated
for multiple overlapping bands.
The SFG signal and the incident ω 1 and ω 2 can be categorized by the direction
of their electric fields with reference to the plane of incidence. Electric field vectors
that lie with the plane of incidence are ‘P-polarized’ and those perpendicular to the
same are known as ‘S-polarized’. S-polarized light contains electric field only along
y-axis; whereas, the P-polarized light contains two electric field components along
x and z axes, respectively (Fig. 1). Polarization of the SFG light entirely depends on
the non-zero χ
(2)
i jk ; selective measurement of a specific component of χ
(2)
i jk is possible
by proper choice of polarization combination of ω 1 , ω 2 and the SFG light. The SFGintensity spectra at different polarization combinations (SSP, SPS, PSS and PPP)
are used to deduce information about orientational/conformational distribution of
interfacial molecules [8].
3 Narrowband Classical-VSFG Spectrometer
Figure 4 shows the typical optical layout of a narrowband classical-VSFG spectrometer. It is called ‘narrowband’ and ‘classical’ to indicate that the IR pulse used for
VSFG has a narrow spectral width and the SFG signal is detected as the conventional intensity spectrum (I SFG ∝ |E SFG |
2 ), respectively. The output from a laser,
such as a mode-locked Nd:YAG laser (∼30 ps, 1064 nm, 50 Hz, 40 mJ/pulse), is
divided into two parts; the first part is frequency doubled to 532 nm with a secondharmonic generator. The 532 nm beam is again split into two; one is used as the
visible pulse (ω V I S ; ∼500 μJ/pulse) for SFG and the other is mixed with the second
part of the 1064 nm fundamental to produce a tuneable infrared pulse in an optical
parametric amplifier and difference frequency generator unit (OPA-DFG unit). The
narrowband IR pulse (ω I R ; ∼260 μJ/pulse @ 3300 cm
−1 ) is spatially and temporally overlapped with ω V I S on the sample surface (e.g., water surface) while the
IR frequency is tuned stepwise to cover the spectral region of a desired vibrational
band. The intensity of the SFG is detected by a single channel detector consist of
a monochromator and photomultiplier tube (PMT). It is obvious that for a vibrational transition in the mid-IR region, the corresponding SFG signal appears in the
visible region (ω I R
3400cm
−1
+ω V I S (532nm) = ω SF (450nm)), and hence, visible
detector such as PMT is used which is easier to handle than that of IR-detector. To
remove the influence of the energy profile of the input IR and visible light, the
measured spectra are normalized with the non-resonant SFG signal from the surface