M. M. T. Loy

Vibrational distributions in desorption induced by femtosecond laser pulses: coupling of adsorbate vibration to substrate electronic excitation

Abstract Quantum-state distributions are reported for nitric oxide (NO) molecules desorbed from a Pd(111) surface at a base temperature of 140 K by laser pulses of 400 fs duration. Significant vibrational populations are observed in the V ; = 0, 1, and 2 levels. For a femtosecond laser fluence capable of producing substrate electronic temperatures of ∼ 4500 K, the vibrational distribution of the desorbed NO molecules is roughly thermal with an average degree of vibrational excitation corresponding to a temperature of 2900 K. These measurements can be regarded as the reverse of the usual vibrational relaxation measurement in that here energy flows from the substrate to the adsorbate vibration. The high degree of excitation of the NO intramolecular vibration can be attributed directly to efficient coupling to the electron-hole pairs generated in the Pd substrate by the femtosecond laser pulse. The process is modeled by calculating electronic and lattice temperatures for the Pd substrate and then incorporating the electron temperature profile into a master equation for the NO vibrational populations. The analysis permits an effective coupling constant of ∼ 15 cm −1 to be inferred for the interaction between the substrate electronic excitation and the intramolecular vibration.

Rotational and electronic relaxation in pulsed supersonic beams of NO seeded in He and Ar

Resonantly enhanced two‐photon ionization has been used to obtain detailed information about the rotational and electronic relaxation in pulsed supersonic expansions of nitric oxide seeded in helium and argon. Due to the sensitivity of the method we were able to measure rotational distributions up to high quantum numbers J″>24.5. Measurements were made for stagnation pressures (u2009p0) and nozzle diameters (d) in the range 0.44≤p0d≤22 [Torru2009cm] for NO/Ar beams, and 0.88≤p0d≤36 [Torru2009cm] for NO/He beams. In general non‐Boltzmann rotational population distributions were observed. Furthermore, we found that for NO/He beams, the two electronic substates 2Π1/2 and 2Π3/2 of NO were not in local equilibrium. These observations can be understood in terms of a simple model using state‐to‐state collision cross sections combined with the empirical hydrodynamic equations to describe the isentropic expansions.

Vibrationally assisted electronic desorption: Femtosecond surface chemistry of O2/Pd(111)

The process of desorption for the system of O2/Pd(111) under excitation by 100 fs pulses of visible light has been examined. Molecular desorption is found to occur with high efficiency and a nonlinear dependence on laser fluence. Direct time‐domain measurements using a two‐pulse correlation scheme reveal a dominant subpicosecond response together with a weaker, but significant correlation persisting for tens of picoseconds. These results imply a desorption process driven by the high electron temperatures produced by the femtosecond laser radiation. The slower component of the correlation response is interpreted as an enhancement of the desorption rate by adsorbate vibrational excitation.

Single quantum state molecular beam scattering of vibrationally excited NO from Ag(111) and Ag(110)

Using an infrared laser excitation/ultraviolet laser detection technique, we have studied the internal energy and state‐specific angular and velocity distributions for a molecular beam of NO(v=1) scattered from single crystal Ag(111) and Ag(110) surfaces in ultrahigh vacuum (UHV). The vibrational energy survival probability is large in both cases, ∼0.9, although the details of the scattering are different for the two surfaces.

Direct observation of rotational cooling in thermal desorption: No/Pd(111)

Abstract The rotational energy distribution of nitric oxide desorbed from a Pd(111) surface has been determined directly at high surface temperature by means of laser heating of the substrate. For a surface temperature of T s = 1100 ± 100 K, the rotational energy distribution of the desorbed NO could be described as thermal, but with a significantly lower temperature of T r = 640 ± 40 K.

Ionization probabilities of A 2Σ+(v′=0,1,2) and B 2Π(v′=0,2) states of NO

Ionization probabilities of NO molecules electronically excited in the Au20092Σ+ and Bu20092Π states have been determined by (1+1) resonance‐enhanced, two‐photon ionization. Various vibrational levels within these states have been excited prior to ionization. Measurements of the unsaturated ionization signal yields accurate values for the relative detection probabilities of NO of 1:(0.70±0.07): (0.67±0.11) for excitation via the γ(0−0), γ(1−1), and γ(2−2) bands, respectively, and (3.7±0.36)×10−7 and (5.8±0.65)×10−4 for ionization through β(0−0) and β(2−1) bands, respectively. Applying published data for the γ‐ and β‐band transition probabilities allows the deduction of the ionization cross section of Au20092Σ+ and Bu20092Π vibrational states. The respective ionization cross sections are (7.0±0.9)×10−19 cm2, (8.5±0.8)×10−19 cm2, (6.0±1.0)×10−19 cm2 for Au20092Σ+(v′=0, 1, and 2) and (5.0±0.5)×10−21 cm2 and (1.7±0.2)×10−20 cm2 for Bu20092Π(v′=0 and 2). These values are based on the experimentally determined cross section for Au20092Σ+(...

Infrared laser multiple photon ionization

The production of ions as a result of the interaction of intense CO2‐ laser pulses with a variety of organic molecules is reported. The laser frequency dependence of the ionization follows closely the linear infrared absorption spectrum of the respective molecule. The extent of ionization depends very strongly on laser energy fluence. The ion‐production process is found to have a strongly collisional character. Various possible mechanisms of laser‐induced ionization are discussed, and evidence is presented favoring laser‐induced chemi‐ionization.

Nonlinear Optical Study of Si Epitaxy

The applicability of the nonlinear optical technique of surface second-harmonic generation to in-situ studies of epitaxial and non-epitaxial crystal growth of centrosymmetric materials is demonstrated. In measurements of the deposition of atomic Si on Si(111)-7×7 surfaces, the (anisotropic) second-harmonic response is seen to be sensitive to the ordering of fractional monolayers of adatoms. For deposition on a substrate held at room temperature, the second-harmonic data are consistent with the formation of a disordered adlayer on top of the original reconstructed surface. The results of real-time measurements of the thermal annealing of disordered Si adlayers of monolayer thickness are also presented.

Pump-Probe Investigation of Femtosecond Desorption

Desorption of NO molecules from a Pd(111) surface has been induced by femtosecond visible laser pulses. Time-resolved measurements of desorption were obtained by means of a correlation technique. The desorption process was found to be characterized by a subpicosecond response, consistent with an electronic mechanism.