A Proposed Method to Obtain Surface Specificity with Pump-Probe and 2D Spectroscopies

Area of Science:

• Surface science
• Spectroscopy
• Materials science

Background:

• Surfaces and interfaces are crucial in biological and material systems, necessitating specialized analytical techniques.
• Sum frequency generation (SFG) spectroscopy, a second-order technique, offers surface specificity by distinguishing surface from bulk signals.
• Third-order spectroscopies lack inherent surface specificity, limiting their application in surface analysis.

Purpose of the Study:

• To propose and validate a novel polarization scheme for third-order spectroscopy to achieve surface specificity.
• To enable selective detection of surface molecular order using third-order techniques.
• To provide an alternative to SFG spectroscopy for surface analysis.

Main Methods:

• Development of a unique polarization scheme for third-order spectroscopy (pump-probe, 2D spectroscopy).
• The scheme employs one perpendicular pulse relative to three others to isolate specific molecular orientations.
• Comparison of the proposed method with SFG spectroscopy and vibrational circular dichroism (VCD).

Main Results:

• The proposed polarization scheme successfully isolates surface signals from bulk signals in third-order spectroscopy.
• Specific polarization combinations (e.g., XXXY, YYYX) reveal distinct cross-peak signals related to surface molecular order.
• The technique demonstrates surface specificity comparable to even-order spectroscopies.

Conclusions:

• The proposed cross-polarized schemes confer surface specificity to odd-ordered spectroscopies.
• This advancement expands the utility of third-order techniques for detailed surface characterization.
• The method offers new insights into molecular behavior at surfaces and interfaces.