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The translational energy of Dz desorbed from Si(100) and Si(111) surfaces was measured and found roughly equal to the thermal expectation at the surface temperature T,. Combining these results with previously measured internal state distributions, the total energy of the desorbed molecules is approximately equal to the equilibrium expectation at T,. Thus adsorption experiments, which suggest a large energetic barrier, are at variance with desorption experiments, which exhibit a trivial adsorption barrier, and the applicability of detailed balance for this system needs to be reexamined.

The control of electron distribution excited in a metal through optical phase of the excitation light is demonstrated. Two-photon photoemission from the Cu(111) surface is excited either by a pair of ϳ15 fs laser pulses with a mutual delay fixed to an accuracy of 60.025 fs, or by a single, frequency-chirped pulse. As a consequence of optical coherence in the two-photon excitation process, the photoemission spectra do not only depend on the frequency, as in conventional spectroscopy, but also on the phase of the excitation light. This may be a general phenomenon in multiphoton ionization.[S0031-9007(97)04735-2] PACS numbers: 73.50. Gr, 78.40.Kc, The reflection of light and flow of current in response to external, time-varying fields are defining electronic properties of metals. An electromagnetic wave with a frequency v , v p (v p is the plasma frequency) is attenuated exponentially at the metal-vacuum interface due to the dynamical response of electrons. The field creates a microscopic polarization at the surface, which can decay by reemission of the field (reflection), or by absorption of a photon (e-h creation). According to the Drude theory, the reflection and absorption of light by a free-electron metal are described, respectively, by the real and imaginary parts of the dielectric constant,´͑v͒ 1 2 v 2 p ͞v͑v 1 i͞t 0 ͒, where t 0 is the optical relaxation time [1,2]. The freecarrier absorption occurs as a second-order process, where a carrier absorbs a photon and simultaneously scatters with a phonon or impurity to conserve momentum. Elastic scattering destroys the phase relation between the excitation created in the sample and the external field. Thus, t 0 is a phenomenological dephasing time. Analysis of freecarrier absorption in noble metals shows that for visible light vt 0 . 1: For example, for Cu at 400 nm (3.1 eV), t 0 is ϳ3.5 fs [2], while an optical cycle is 1.33 fs. Thus, the scattering processes described by t 0 do not present a fundamental limit for controlling quantum dynamical response of electrons in metals by means of the optical phase.Coherent control of quantum dynamics is a rapidly developing field of physical sciences [3]. Demonstration of control of quantum dynamics in atoms [4], molecules [5,6], molecular crystals [7], and semiconductors [8] has been achieved with phase-engineered light pulse sequences and chirped pulse excitation [3]. Interference between oneand two-photon excitation has been used for control of electrical currents in semiconductors [9] and the direction of photoemission from a surface [10]. Optical control of electron dynamics in metals and at metallic interfaces also is of great interest in a variety of fields including solid state physics, surface science, and for applications in optoelectronics. However, the dephasing time implied by t 0 , which is less than the currently available laser pulse widths, seems to impose severe limits for demonstration and application of coherent control in metals. Nevertheless, development of ultrafast interferometric techniques for st...

The adsorption of D2 on Si(100) has been investigated by means of supersonic molecular beam techniques. We have succeeded in measuring the dependence of the molecular D2 sticking coefficient S on surface temperature Ts and nozzle temperature Tn. The sticking coefficient increases gradually in the range 300≤Tn≤1040 K. The influence of increased v=1 population has not been deconvoluted from the effects of translational energy alone. The dependence on Ts is more interesting. With an incident translational energy of 65 meV, S rises from a value insignificantly different from the background level to a maximum value of (1.5±0.1)×10−5 at Ts=630 K. The decrease in the effective sticking coefficient beyond this Ts is the result of desorption during the experiment. Having established that S increases with both increasing molecular energy and increasing sample temperature, we have demonstrated directly for the first time that the adsorption of molecular hydrogen on Si is activated and that lattice vibrational excitations play an important role in the adsorption process.

Electron lifetimes in a high-T c cuprate Bi 2 Sr 2 CaCu 2 O 81d are measured directly in time domain by the interferometric two-photon time-resolved photoemission technique. The lifetimes decrease from 60 to 10 fs in an energy range of 1.4 to 3.0 eV above E F with approximately ͑E 2 E F ͒ 22 energy scaling of a normal, Fermi liquid metal. However, anomalous behavior shows up in a pronounced temperature dependence where the lifetimes of the 1.6 eV electrons decrease by 30% from 40 to 340 K. This study helps to define the nature of inelastic carrier scattering processes in the normal state of high-T c cuprates. [S0031-9007(98)07695-9] PACS numbers: 74.25.Jb, 73.50.Gr, 74.72.Bk, 78.47. + p Despite more than ten years of intense research, the type of interaction among electrons responsible for the high temperature superconductivity is still an open issue. Undoped parent compounds of the cuprate high temperature superconductors (HTSC) are charge transfer insulators, whose electronic structure and macroscopic bulk properties are dominated by the Coulomb interaction and magnetic effects [1]. Upon doping these materials become metallic, but their normal state properties, such as the linear dependence of electrical resistivity and optical conductivity, respectively, on temperature and frequency are anomalous [2]. In normal metals, the ͑E 2 E F ͒ 22 scaling of the e-e scattering times, which is predicted by the Fermi liquid theory (FLT), gives a quadratic T ͞v dependence of quasiparticle lifetimes that has not been observed in HTSCs [3][4][5]. Commonly used methods for studying the carrier interactions in HTSCs, such as electrical resistivity [6], optical conductivity [7], and photoemission spectroscopy [8], probe the momentum scattering rates. Momentum randomization can occur through a variety of processes such as quasielastic scattering between the carriers and phonons, or spin fluctuations, as well as inelastic scattering involving the carrier decay into single particle or collective electronic excitations [2]. However, the most direct way to test the Fermi liquid properties of the normal state of cuprates is to specifically probe the energy dependence of the e-e scattering rates.An alternative way to study charge carrier dynamics in solid state materials is through time-domain techniques. Previous time-domain studies on HTSCs probed the ultrafast optical response through femtosecond transient absorption and/or reflectivity measurements [9][10][11][12]. The focus has been mainly on the dynamics of Cooper pair breaking and reformation as a result of impulsive laser heating and subsequent equilibration of the electronic system. These studies report a fast (60-350 fs), sample dependent, transient signal at early times, which is assigned to the thermalization of the nascent nonequilibrium carrier distribution. Since the optical constants of a metal depend on the Fermi distribution in a complex manner, it has not been possible to extract specific energy resolved scattering rates, or to elucidate the relaxation mechanism. ...