Hisashi Nagano

Ultrafast interferometric pump–probe correlation measurements in systems with broadened bands or continua

Ultrafast (femtosecond) interferometric pump–probe techniques can be used to measure rates of population and quantum phase decay in complicated media such as liquids and solids. However, the levels probed in such systems are often inhomogeneously broadened or are part of a continuum of states. The use of broadband ultrafast lasers thus results in multiple levels being excited and detected. The inherent averaging that is due to this effect can alter the measured coherent response, thus affecting the information that can be retrieved on the phase decay. The importance of these effects is considered for the representative case of two-photon photoemission from metals. The effects of (i) continuum excitation; (ii) excitation from the Fermi level, i.e., a spectral step function; (iii) excitation from broadened levels with a finite width; and (iv) photoelectron energy analyzer resolution are determined.

The role of Auger decay in hot electron excitation in copper

The role of different excitation mechanisms in two-photon photoemission measurements of the hot electron population dynamics in copper is considered. The effective hot electron lifetimes derived from two-pulse correlation measurements with ∼3.1–3.8 eV, 50 fs laser pulse excitation are different depending on whether the hot electrons are generated by interband d→sp or intraband sp→sp excitation (S. Pawlik, M. Bauer, M. Aeschlimann, Surf. Sci. 377–379 (1997) 206). A proposed explanation is that the latter process actually occurs by the Auger recombination of long-lived d-band holes resulting in complex hot electron population dynamics involving this delayed generation process and decay by the electron–electron scattering [E. Knoesel, A. Hotzel, M. Wolf, Phys. Rev. B 57 (1998) 12812]. This proposal is tested by simulation of interferometric two-pulse correlation measurements on the low index surfaces of copper (Cu(111), (100), and (110)) by the optical Bloch equations. The lower limit for the d-hole lifetime due to the Auger recombination of 24±3 fs for modeling of how this generation process affects the hot electron population kinetics is established from the d-hole decoherence measurements at the X5 point. Optical Bloch equation fits of the data show that at most <10% of hot electrons at 1.4 eV are generated through a secondary generation mechanism, therefore Auger recombination cannot explain the anomalous hot electron population dynamics.

Sensitive Monitoring of Volatile Chemical Warfare Agents in Air by Atmospheric Pressure Chemical Ionization Mass Spectrometry with Counter-Flow Introduction

A new method for sensitively and selectively detecting chemical warfare agents (CWAs) in air was developed using counter-flow introduction atmospheric pressure chemical ionization mass spectrometry (MS). Four volatile and highly toxic CWAs were examined, including the nerve gases sarin and tabun, and the blister agents mustard gas (HD) and Lewisite 1 (L1). Soft ionization was performed using corona discharge to form reactant ions, and the ions were sent in the direction opposite to the airflow by an electric field to eliminate the interfering neutral molecules such as ozone and nitrogen oxide. This resulted in efficient ionization of the target CWAs, especially in the negative ionization mode. Quadrupole MS (QMS) and ion trap tandem MS (ITMS) instruments were developed and investigated, which were movable on the building floor. For sarin, tabun, and HD, the protonated molecular ions and their fragment ions were observed in the positive ion mode. For L1, the chloride adduct ions of L1 hydrolysis products were observed in negative ion mode. The limit of detection (LOD) values in real-time or for a 1 s measurement monitoring the characteristic ions were between 1 and 8 μg/m(3) in QMS instrument. Collision-induced fragmentation patterns for the CWAs were observed in an ITMS instrument, and optimized combinations of the parent and daughter ion pairs were selected to achieve real-time detection with LOD values of around 1 μg/m(3). This is a first demonstration of sensitive and specific real-time detection of both positively and negatively ionizable CWAs by MS instruments used for field monitoring.

High-throughput walkthrough detection portal for counter terrorism: detection of triacetone triperoxide (TATP) vapor by atmospheric-pressure chemical ionization ion trap mass spectrometry.

With the aim of improving security, a high-throughput portal system for detecting triacetone triperoxide (TATP) vapor emitted from passengers and luggage was developed. The portal system consists of a push-pull air sampler, an atmospheric-pressure chemical ionization (APCI) ion source, and an explosives detector based on mass spectrometry. To improve the sensitivity of the explosives detector, a novel linear ion trap mass spectrometer with wire electrodes (wire-LIT) is installed in the portal system. TATP signals were clearly obtained 2 s after the subject under detection passed through the portal system. Preliminary results on sensitivity and throughput show that the portal system is a useful tool for preventing the use of TATP-based improvised explosive devices by screening persons in places where many people are coming and going.

High-Throughput Walkthrough Detection Portal as a Measure for Counter Terrorism: Design of a Vapor Sampler for Detecting Triacetone Triperoxide Vapor by Atmospheric-Pressure Chemical-Ionization Ion-Trap Mass Spectrometry

Aiming to prevent terrorist attacks in places where many people are coming and going, we have been developing a “high-throughput detection portal system.” The portal system consists of a vapor sampler, an atmospheric-pressure chemical-ionization ion source, and an explosives detector based on ion-trap mass spectrometry. The vapor sampler was designed to be installed in an automated ticket gate of a train station. By optimizing the shape of the nozzle that controls the air flow of the vapor sampler, triacetone triperoxide (TATP) vapor could be detected at a high throughput, i.e., 1200 persons/hour. The false-positive rate of the detection portal system for TATP was evaluated by a field test performed at a train station. A multi-marker logic to determine whether TATP existed or not was adopted, and no false-positive alarms were obtained for over 3000 passengers during the field test. However, acetone, which is an inflammable liquid, was accidentally detected from the passengers during the field test. It is concluded from this detection result that this detection portal system is useful for detecting dangerous chemicals that have high vapor pressure (such as TATP and inflammable liquids) in places where many people are coming and going.

Energy relaxation and dephasing times of excited electrons in Bi2Sr2CaCu2O8+δ from interferometric 2-photon time-resolved photoemission

Abstract The electron dynamics inBi 2 Sr 2 CaCu 2 O 8+δ are investigated with interferometric 2-photon time-resolved photoemission. By decomposing the interferometric 2-pulse correlation into oscillating and non-oscillating components energy relaxation times can be obtained for excited electrons above the Fermi level, T 1 , and dephasing times of electron-hole pairs, T 2 . The procedure to determine T 1 and T 2 is discussed and first results on the electron energy relaxation time in a high temperature superconductor are presented.

Optical decoherence and quantum beats in Cs/Cu(111)

Electron dynamics induced by optical excitation from the occupied Shockley surface state to an unoccupied state localized on Cs atoms on Cu(111) is studied by interferometric time-resolved two-photon photoemission. With near-resonant excitation, beat structures are observed in interferometric two-pulse correlation measurements, where the two-photon photoemission intensity is measured as a function of pump-probe delay time. The quantum beats that originate from the frequency differences among initial-intermediate and intermediate-final states, and the carrier frequency are reproduced qualitatively by a three-level optical Bloch equation model.

Mass spectra separation for explosives detection by using probabilistic latent component analysis

We propose a new method to separate mass spectra into components of each chemical compound for explosives detection. In mass spectra, all components have no negative values. However, conventional factor analyses for basis decomposition use no constraints of non-negativity, and we can not apply these methods to mass spectra. The proposed method is based on probabilistic latent component analysis (PLCA). The constraints of non-negativity always hold in PLCA, so that the method is effective for mass spectra. In addition, PLCA is defined in a statistical framework, thus PLCA makes it possible to utilize additional a priori information. Therefore, we introduce sparseness assumptions in the domain of mass spectrometry to PLCA in order to estimate the components more accurately. Experimental results indicate that the proposed method outperforms existing methods.

Real‐time explosive particle detection using a cyclone particle concentrator

RATIONALE There is a need for more rapid methods for the detection of explosive particles. We have developed a novel real-time analysis technique for explosive particles that uses a cyclone particle concentrator. This technique can analyze sample surfaces for the presence of particles from explosives such as TNT and RDX within 3 s, which is much faster than is possible by conventional methods. METHODS Particles are detached from the sample surface with air jet pulses, and then introduced into a cyclone particle concentrator with a high pumping speed of about 80 L/min. A vaporizer placed at the bottom of the cyclone particle concentrator immediately converts the particles into a vapor. The vapor is then ionized in the atmospheric pressure chemical ionization (APCI) source of a linear ion trap mass spectrometer. RESULTS An online connection between the vaporizer and a mass spectrometer enables high-speed detection within a few seconds, compared with the conventional off-line heating method that takes more than 10 s to raise the temperature of a sample filter unit. Since the configuration enriched the number density of explosive particles by about 80 times compared with that without the concentrator, a sub-ng amount of TNT particles on a surface was detectable. CONCLUSIONS The detection limit of our technique is comparable with that of an explosives trace detector using ion mobility spectrometry. The technique will be beneficial for trace detection in security applications, because it detects explosive particles on the surface more speedily than conventional methods.

Improvement of robustness to change of positive elements in boolean compressive sensing

A new boolean compressive sensing method for solving the group-testing problem is proposed. The conventional method has the problem that the estimation performance is degraded in the case that positive elements change in the middle of tests because the results of the tests before a change-point are inconsistent with those of the tests after the change-point. To solve the problem, the proposed method detects the latest change-point of positive elements, and it finds positive elements by using only the results of the tests after the change-point. To detect the change-point, the proposed method makes use of the fact that the distribution of the results depends on the number of positive elements. Experimental simulation indicates that the proposed method outperforms the conventional method on the condition that positive elements change in the middle of tests.