(a) Experimental arrangement for vibrational SFG spectroscopy of SAMs adsorbed on Au films. In the depiction, SFG probes CH-stretch transitions of the terminal methyl groups (circled) of an ODT SAM. BBIR and NBVIS are broadband IR and narrowband visible pulses. The Cr adhesion layer keeps Au from debonding during solution deposition of SAMs. (b) Lissajous pattern xy scanning of the substrate minimized accumulated optical damage by reducing multishot exposure.
Reflectance changes of Au substrates heated to indicated temperatures. Although 530 nm has the largest reflectance change, the 600 nm portion of the white-light continuum was used for transient temperature determinations due to its greater intensity.
Transient reflectance changes from Au films flash-heated from ambient temperature by 200 fs 400 nm (blue) pulses. The longer-time (>20 ps) reflectance change at 600 nm gives ΔT = 175 K. Note the nonlinear time axis.
Reflectance transients at 600 nm from flash-heated Au films. ΔT was determined from ΔR/R in the plateau region using the calibration Eq. (1) . (a) A smaller-amplitude T-jump where ΔT = 35 K was produced by either 800 nm or 400 nm flash-heating pulses. The electron-phonon equilibration time constant was 1.25 ps with 400 nm and 1.65 ps with 800 nm pulses. Adsorbed SAMs had no effect. (b) A T-jump transient with ΔT = 175 K produced with 400 nm pulses. The time constant for electron-phonon equilibration increased to 3.45 ps.
Results of flash-heating experiments where SFG probed νs(NO2) of NBT SAMs on Au substrates flash-heated by either 800 nm or 400 nm pulses. When the same ΔT = 35 K was produced, the NBT transients were identical within experimental error.
Results of flash-heating experiments with ΔT = 175 K where SFG probed νs(NO2) of NBT SAM on Au. (a) SFG spectra at indicated delay times. (b) Zeroth spectral moment M ( 0 ), representing the intensity (integrated peak area) loss induced by flash-heating. (c) First moment M ( 1 ) representing lineshape redshift. (d) Square-root of the second moment M ( 2 ) representing linewidth.
(a) Time-dependent SFG spectra of ODT SAM on Au at indicated delay times after flash-heating by 400 nm pulses with ΔT = 175 K. The three main SFG transitions arise from ODT terminal methyl groups (see Fig. 1 ). (b) SFG spectrum of ODT after ∼1 h of signal averaging with the substrate moving in a Lissajous pattern. The effects of more than 1 × 106 laser pulses were negligible. (c) Changes in ODT SFG spectrum after a stationary sample was exposed to 103 flash-heating pulses with ΔT = 115 K. Exposure caused SFG signal loss. When the two spectra were normalized to facilitate comparisons, new transitions indicated by the arrows appeared. The new transitions are indicative of alkyl chains developing gauche defects. The defects were created by the cumulative effects of many large-amplitude T-jumps.
(a) VRF for ODT flash-heated by 400 nm pulses that produced ΔT = 175 K was characterized by an onset delay t 0 and a biexponential decay with time constants τ1 and τ2. (b) Time dependent changes in the SFG intensity ratio νs(CH3)/νas(CH3) induced by smaller and larger T-jumps. (c)Definition of the methyl tilt angle θ. (d) Relative amplitudes of νs(CH3) and νas(CH3) as a function of ensemble-averaged methyl tilt angle θ, based on Hirose et al. 55,56
(a) Schematic of the NBT SAM ordered structure. Arrows indicate conformational degrees of freedom leading to nitro group disorder. (b) Vibrational excitations of NBT resulting from optical pumping of the Au layer. Hot electrons produced initially can excite higher-frequency vibrations including the probed νs(NO2) and other vibrations not probed. As hot electrons decay and the Au lattice temperature rises from Tcold to Thot , lower-energy NBT SAM lattice modes and NBT vibrations become excited by multiphonon up-pumping. (c) Schematic of SFG process for a vibration initially in v = 0, with coherent IR excitation followed by coherent anti-Stokes Raman scattering. When the probed vibration is excited into v = 1 by hot electrons, the SFG signal decreases due to ground-state depletion. (d) Schematic of vibrational energy exchange mechanism used to explain thermally induced redshifting and broadening. The notation |nm> indicates n quanta in the probed mode and m quanta in the other mode. When νs(NO2) is probed and another anharmonically coupled vibration becomes excited, the new probed transition is redshifted by δν. The lifetime of the coupled vibration is τ and the extent of broadening is determined by the product δντ.
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