Nanoshocks and surfaces:  fracture and lubrication
Home page Contact us Directions for visitors Research Projects Dana D. Dlott (Curriculum Vitae) Dana D. Dlott (personal home page) David M. Dlott (personal home page) Group Members Research Publications Research Funding Research Facilities	In the nanoshock CARS apparatus, the sample layer thickness is generally in the 0.2-0.7 micron range.  The time resolution increases as the sample thickness decreases, but it is difficult to fabricate samples thinner than 0.2 micron and the CARS signal becomes very small.  In order to obtain the ultimate time resolution, the sample should be a monolayer and the probe technique should be a surface-selective spectroscopic probe. In recent years, vibrational sum-frequency generation (SFG) has been developed.  As shown at right, SFG experiments are performed by combining a vibrational infrared pulse and a visible pulse at a surface.  The SFG signal becomes more intense when the IR pulse is resonant with a surface vibrational transition.  The SFG signal is not generated in bulk centrosymmetric media, so the technique is surface-selective.  In the broadband multiplex configuration shown at right, a broad band IR (BBIR) pulse is generated, whose spectrum covers several surface vibrational transitions.  It is combined with a narrow band visible pulse (NBvis).  When the SFG signal is frequency resolved by an optical array (CCD), an entire spectrum is obtained on every laser shot.  The combination of nanoshocks and SFG is used to obtain very high time resolution of shock wave effects in molecules.  Our initial experiments used an alkane self-assembled monolayer (SAM). SFG is used to probe CH stretching transitions of the methyl head groups. Using polarization spectroscopy we can determine the instantaneous orientation of the methyl group. This orientation parameter indicates whether the SAM chains are tilted or whether the shock causes them to isomerize. In the near future we intend to use this technique to look at thin layers of water confined at hydrophilic and hydrophobic interfaces and also CO molecules on metal surfaces. References “Shock compression of molecules with 1.5 angstrom resolution”, James E. Patterson, Alexi Lagoutchev and Dana D. Dlott, AIP Confer. Proc. vol. 706 (2004), pp. 1299-1302. “Ultrafast dynamics of shock compression of molecular monolayers”, James E. Patterson, Alexei Lagutchev, Wentao Huang and Dana D. Dlott, Phys. Rev. Lett. 94, 015501 (2005). This paper was selected to appear in the “Virtual Journal of Ultrafast Science” “Ultrafast Dynamics of Self-Assembled Monolayers Under Shock Compression: Effects of molecular and substrate structure”, Alexei Lagoutchev, James E. Patterson, Wentao Huang and Dana D. Dlott, J. Phys. Chem. B 109, pp. 5033-5044 (2005). “Ultrafast shock compression of self-assembled monolayers: a molecular picture”, James E. Patterson and Dana D. Dlott, J. Phys. Chem. B 109, pp. 5045-5054 (2005). “Time and space resolved studies of shock compression molecular dynamics”, J. E. Patterson, A. S. Lagutchev, S. A. Hambir, W. Huang, H. Yu, and Dana D. Dlott, Shock Waves (in press). “Shock compression spectroscopy with high time and space resolution”, Wentao Huang, James E. Patterson, Alexei Lagutchev and Dana D. Dlott, AIP Confer. Proc. (in press).	Layout for broadband multiplex vibrational sum-frequency generation (SFG).  BB-broadband; NB-narrowband Femtosecond laser with shock generation and SFG probing Shock dynamics with femtosecond time resolution and atomic space resolution. When the femtosecond laser-driven shock passes over the terminal methyl of the self-assembled monolayer, SFG probes the C-H stretching vibrations. Dr. Alexi Lagoutchev Dr. Wentao Huang