Femtosecond surface science

We are interested in the fundamental physical processes which govern laser-induced chemical reactions at solid surfaces. At metal surfaces the light can promote a reaction by stimulating the electrons in the metal. These electrons can interact with molecules on the surface of the metal to induce chemical reactions. Our goal is to understand the exchange of energy between the metal substrate and the reactant molecules on the surface. A particular challenge is to time-resolve the reaction path, identifying extremely rapid processes which occur while the molecules are in transition states. The tools available are a mass spectrometer and ultrafast-laser pulses to measure reaction products and induce electronic excitations respectively.

It is difficult to identify the transient reactive species in chemical reactions at surfaces. The reactive species itself is believed to have complicated electronic and vibrational modes of excitation which interact strongly with the substrate. What is the nature of the reactive species? How do energetic electrons in the substrate interact with the adsorbate? On what time-scale is energy transferred to the adsorbate? Is it possible to promote one reaction path over another?

We clean a metal surface in ultra-high vacuum and then cover it with a layer of small molecules. We start a chemical reaction with a laser pulse from a regeneratively-amplified Ti-Sapphire laser. The reaction products are measured with a mass spectrometer. We infer the reaction path by studying the reaction yield as a function of the laser pulse duration, central wavelength, and energy per unit area. A 'two-pulse correlation' experiment, in which two laser pulses strike the surface one after another, is used to study the lifetime of the excitations.

Femtosecond laser doping of TiO2 for photocatalysis, at Photonics West (San Fransisco, CA), Monday, January 24, 2011:
We present a novel method for femtosecond-laser doping of titanium dioxide (TiO2) for enhanced absorptance in the visible electromagnetic spectrum. With a bandgap of 3.2 eV for the anatase crystalline phase, TiO2 most strongly absorbs in the UV range ( < 387 nm). However, doping with metals and nitrogen has been shown to create intermediate states in the bandgap, generating a new material for visible-light photocatalysis that has the potential for watersplitting. Using femtosecond laser doping techniques on bulk TiO2 in a gaseous environment, we produce laser-induced periodic surface... Read more about Femtosecond laser doping of TiO2 for photocatalysis
Surface femtochemistry: Elucidating reaction pathways, at Ultrafast Surface Dynamics Workshop (Ascona, Switzerland), Wednesday, March 5, 1997:

We studied the desorption of O2 and production of CO2 induced by intense femtosecond laser pulses incident on a Pt(111) surface prepared with coadsorbed CO and O2 at 90 K. The reaction pathway with femtosecond laser pulses is very different from that with nanosecond or contineous wave irradiation [1,2]. Our experiments address both the excitation mechanism as well as the chemical pathway leading to O2 desorption and CO2...

Read more about Surface femtochemistry: Elucidating reaction pathways
Surface photochemistry of O2 on Pt(111) with femtosecond laser pulses, at 40th American Vacuum Society Conference (Orlando, FL), Friday, January 1, 1993
Previous studies of O2 desorption and photoreaction from Pt(111)1 and Pd(111)2 upon irradiaiton with intense (>1mJ/cm2) 100fs laser pulses have observed highly nonlinear fluence dependences with power law exponents of 5 and higher. It has been proposed that multiple electronic transitions induce desorption either via coupling to transient high energy substrate electrons3 or more direct excitation.4
Electron-beam-induced oxidation of benzene to phenol in C6H6/O2/Pt(111), at APS Centennial Meeting 1999 (Atlanta, GA), Tuesday, March 23, 1999:
We have observed the formation of phenol when C6H6/O2/Pt(111) is exposed to a beam of electrons with energy on the order of 10-100 eV. With this sample preparation, the oxygen is bound directly to the platinum; the benzene overlayer does not displace the oxygen and chemisorb to the platinum. In contrast, no electron-beam-induced phenol production is observed when the benzene and oxygen are applied in the reverse order, for O2/C6H6/Pt(111). Following exposure to the electron beam, the yield of phenol is measured during temperature-programmed desorption. In the absence of the electron beam, no... Read more about Electron-beam-induced oxidation of benzene to phenol in C6H6/O2/Pt(111)
Surface photochemistry of O2 on Pt(111) with femtosecond laser pulses, at 40th American Vacuum Society Conference (Orlando, FL), Friday, January 1, 1993
Previous studies of O2 desorption and photoreaction from Pt(111)1 and Pd(111)2 upon irradiaiton with intense (>1mJ/cm2) 100fs laser pulses have observed highly nonlinear fluence dependences with power law exponents of 5 and higher. It has been proposed that multiple electronic transitions induce desorption either via coupling to transient high energy substrate electrons3 or more direct excitation.4
Femtosecond laser production of TiO2, at Hyperdoping Research Meetup, MIT (Cambridge, MA), Friday, April 15, 2011:
We present a novel method for producing TiO2 through femtosecond-laser processing titanium in the presence of oxygen. The process produces laser-induced periodic surface structures that are consistent with previous work done on titanium. We compare how the surface morphology and composition vary with gas composition and laser parameters. Using x-ray photoelectron and Raman spectroscopy, we will show that chemical selectivity plays an important role in hyperdoping titanium. With this method, we hope to introduce dopants, such as chromium and nitrogen, into the lattice for visible light... Read more about Femtosecond laser production of TiO2
Femtosecond surface photochemistry: what is the role of the substraste electrons?, at Femtochemistry: The Lausanne Conference (Lausanne, Switzerland), Thursday, September 7, 1995
We studied the femtosecond laser-induced desorption of O2 and production of CO2 from a CO/O2/Pt(111) surface at 90K. The reaction pathway with 0.3-ps laser pulses is very different from that with nanosecond or continuous wave irradiation.[1,2] Our experiments address both the excitation excitation mechanism and the chemical pathway leading to O2 desorption and CO2 production using femtosecond laser pulses. When the fluence is below 10 µJ/mm2 the yields of O2 and CO2 vary linearly with laser fluence. Above 10 µJ/mm2 the yield is highly nonlinear in the fluence. The transition indicates a... Read more about Femtosecond surface photochemistry: what is the role of the substraste electrons?

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