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We report subpicosecond electro-optic measurements of the length of individual relativistic electron bunches. The longitudinal electron-bunch shape is encoded electro-optically on to the spectrum of a chirped laser pulse. The electron-bunch length is determined by analyzing individual laser-pulse spectra obtained with and without the presence of an electron bunch. Since the length of the chirped laser pulse can be easily changed, the electron bunch can be visualized on different time scales. This single-shot imaging technique is a promising method for real-time electron-bunch diagnostics.

The cavity enhanced absorption (CEA) technique is extended into the 10 mum region using a line-tunable continuous wave CO2 laser. Part of the laser beam is deflected by an acousto-optical modulator (AOM), and is used to excite a mechanically unstable high-finesse optical cavity. In order to assure a stable and optimal transmittance of light through the cavity, the laser frequency and the cavity eigenfrequencies are modulated independently. The time-integrated intensity of the light exiting the cavity, which is inversely proportional to the cavity losses, is measured using a lock-in detection scheme. An absorption detection sensitivity of 1.5 x 10(-6) cm(-1) Hz(-1/2) is readily obtained with a rather simple setup. (C) 2001 Elsevier Science B.V. All rights reserved.

Cavity ring-down (CRD) spectroscopy is a direct absorption technique, which can be performed with pulsed or continuous light sources and has a significantly higher sensitivity than obtainable in conventional absorption spectroscopy. The CRD technique is based upon the measurement of the rate of absorption rather than the magnitude of absorption of a light pulse confined in a closed optical cavity with a high Q factor. The advantage over normal absorption spectroscopy results from, firstly, the intrinsic insensitivity to light source intensity fluctuations and, secondly, the extremely long effective path lengths (many kilometres) that can be realized in stable optical cavities. In the last decade, it has been shown that the CRD technique is especially powerful in gas-phase spectroscopy for measurements of either strong absorptions of species present in trace amounts or weak absorptions of abundant species. In this review, we emphasize the various experimental schemes of CRD spectroscopy, and we show how these schemes can be used to obtain spectroscopic information on atoms, molecules, ions and clusters in many environments such as open air, static gas cells, supersonic expansions, flames and discharges.

A series of pulsed electric fields can be arranged such that it creates a traveling potential well in which neutral dipolar molecules can be confined. This provides a method to transport, to decelerate, and to cool a sample of neutral molecules while maintaining the initial phase-space density. This method is described using the concept of phase stability. The oscillating motion of molecules in the traveling potential well, reaching a minimum velocity spread corresponding to a translational temperature of 4 mK, is experimentally observed.

The k3π state of the CO molecule is investigated in the region between 91 000 and 97 000 cm−1 via 1+1 resonance enhanced multiphoton ionization spectroscopy on CO molecules prepared in a single quantum level of the a3π(v=1) state. A new vibronic band is found which is at lower energy than the vibrational ground state reported in the literature, leading to a reassignment of the vibrational numbering of the k3π state. The rotationally resolved spectra of the k3π (v=0–6)←a3π(v=1, J=1, Ω=1) of 12CO and 13CO have been observed and analyzed, confirming the new vibrational labeling and providing a full set of molecular constants of the k3π valence state.

Experiments are performed in which vibrationally and electronically excited CO(a 3 II, v = 1) molecules are scattered from a LiF(100) surface. As there is originally no population in the vibrationless level of the metastable state, this experiment gives the unique possibility of probing the vibrationally inelastic channel in the scattering of vibrationally excited molecules. The vibrational deactivation probability can thus be accurately determined and is found to be below 10 −3 for the system under study.

A scheme is presented for the confinement of neutral molecules in stable orbits on the basis of the linear Stark effect in a properly designed electrostatic trap. Based on Stark shift measurements in electric fields up to 140 kV/cm, an electrode configuration which is, under realistic experimental conditions, capable of confining metastable CO(a31-1) molecules with velocities up to 22 m/s is described. Metastable CO molecules can be laser-prepared inside the trap and their trajectories can be directly visualized via the spatially resolved detection of their fluorescence. (~) 1997 Elsevier Science B.V.

Survival of electronically excited, state-selected $\mathrm{CO}({a}^{3}\ensuremath{\Pi})$ molecules scattering from a cleaved LiF(100) surface is studied. Absolute survival probabilities are determined for various impact energies. Time-of-flight profiles, angular distributions, and internal state distributions of the surviving metastable CO molecules are measured. The dependence of their translational and rotational temperatures on the incoming velocity unambiguously indicates a direct inelastic scattering process even though the angular distributions are broad, both in plane and out of plane.

In our Letter [Opt. Lett. 31, 1753 (2006)], we derived, in a very general way, the temporal electric field modifications due to the chi((2)) electro-optic effect; to maintain generality we did not seek to narrow our result to specific experimental conditions, nor did we make any assumptions about birefringence or polarization states. A description of specific experimental conditions can be simply obtained by applying an appropriate Jones matrix calculation. The results noted in the Comment of Yellampalle et al. [Opt. Lett. 32, 1341 (2007)] are no more than a particular application of our general results. (C) 2007 Optical Society of America

Contains fulltext : 33124.pdf (Publisher’s version ) (Open Access) We report the observation at high resolution of seven vibronic bands that appear within similar to200 cm(-1) of the electronic origin in the S-1-S-0 fluorescence excitation spectrum of 4,4(')-dimethylaminobenzonitrile (DMABN) in a molecular beam. Surprisingly, each band is found to be split into two or more components by a (coordinated) methyl group tunneling motion which significantly complicates the analysis. Despite this fact, high quality [(Observed-Calculated)less than or equal to30 MHz] fits of each of the bands have been obtained, from which the rotational constants, inertial defects, torsion-rotation interaction constants, methyl group torsional barriers, and transition moment orientations of DMABN in both electronic states have been determined. The data show that DMABN is a slightly pyramidalized (similar to1degrees) but otherwise (heavy-atom) planar molecule in its ground S-0 state, and that its electronically excited S-1 state has both a more pyramidalized (similar to3degrees) and twisted (similar to25degrees) dimethylamino group. Large reductions in the methyl group torsional barriers also show that the S-1

The dinuclear copper complex bearing a 2,7-disubstituted-1,8-naphthalenediol ligand, [(HtomMe){Cu(OAc)}2](OAc), a potential anticancer drug able to bind to two neighboring phosphates in the DNA backbone, is endowed with stronger cytotoxic effects and inhibition ability of DNA synthesis in human cancer cells as compared to cisplatin. In this study, the intrinsic binding ability of the charged complex [(HtomMe){Cu(OAc)}2]+ is investigated with representative phosphate diester ligands with growing chemical complexity, ranging from simple inorganic phosphate up to mononucleotides. An integrated method based on high-resolution mass spectrometry (MS), tandem MS, and infrared multiple photon dissociation (IRMPD) spectroscopy in the 600-1800 cm-1 spectral range, backed by quantum chemical calculations, has been used to characterize complexes formed in solution and delivered as bare species by electrospray ionization. The structural features revealed by IRMPD spectroscopy have been interpreted by comparison with linear IR spectra of the lowest-energy structures, revealing diagnostic signatures of binding modes of the dinuclear copper(II) complex with phosphate groups, whereas the possible competitive interaction with the nucleobase is silenced in the gas phase. This result points to the prevailing interaction of [(HtomMe){Cu(OAc)}2]+ with phosphate diesters and mononucleotides as a conceivable contribution to the observed anticancer activity.

Deamidation is a major fragmentation channel upon activation by collision induced dissociation (CID) for protonated peptides containing glutamine (Gln) and asparagine (Asn) residues. Here, we investigate these NH3-loss reactions for four Asn- and Gln-containing protonated peptides in terms of the resulting product ion structures using infrared ion spectroscopy with the free electron laser FELIX. The influence of the side chain length (Asn versus Gln) and of the amino acid sequence on the deamidation reaction has been examined. Molecular structures for the product ions are determined by comparison of experimental IR spectra with spectra predicted by density functional theory (DFT). The reaction mechanisms identified for the four dipeptides AlaAsn, AsnAla, AlaGln, and GlnAla are not the same. For all four dipeptides, primary deamidation takes place from the amide side chain (and not from the N-terminus) and, in most cases, resembles the mechanisms previously identified for the protonated amino acids asparagine and glutamine. Secondary fragmentation reactions of the deamidation products have also been characterized and provide further insight in – and confirmation of – the identified mechanisms. Overall, this study provides a comprehensive molecular structure map of the deamidation chemistry of this series of dipeptides. Graphical Abstractᅟ Electronic supplementary material The online version of this article (doi:10.1007/s13361-016-1462-5) contains supplementary material, which is available to authorized users.

Agrochemicals frequently undergo various chemical and metabolic transformation reactions in the environment that often result in a wide range of derivates that must be comprehensively characterized to understand their toxicity profiles and their persistence and outcome in the environment. In the development phase, this typically involves a major effort in qualitatively identifying the correct chemical isomer(s) of these derivatives from the many isomers that could potentially be formed. Liquid chromatography-mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy are often used in attempts to characterize such environment transformation products. However, challenges in confidently correlating chemical structures to detected compounds in mass spectrometry data and sensitivity/selectivity limitations of NMR frequently lead to bottlenecks in identification. In this study, we use an alternative approach, infrared ion spectroscopy, to demonstrate the identification of hydroxylated derivatives of two plant protection compounds (azoxystrobin and benzovindiflupyr) contained at low levels in tomato and spinach matrices. Infrared ion spectroscopy is an orthogonal tandem mass spectrometry technique that combines the sensitivity and selectivity of mass spectrometry with structural information obtained by infrared spectroscopy. Furthermore, IR spectra can be computationally predicted for candidate molecular structures, enabling the tentative identification of agrochemical derivatives and other unknowns in the environment without using physical reference standards.

Cavity ring down absorption spectroscopy with a continuous light source is used to measure the transition frequencies and absolute absorption coefficient of the weak b1Σg+(v′ = 2) ← X 3Σg−(v″ = 0) transition of 18O2. The absorption spectrum is extracted from a measurement of the magnitude of the phase shift that an intensity modulated continuous light beam experiences upon passing through an unstabilized optical cavity.

A compact diode laser operating around 1.5 mum was used to measure cavity enhanced absorption spectra of hot water molecules and OH radicals in radiative environments under atmospheric conditions. Spectra of air were measured in an oven at temperatures ranging from 300K to 1500 K. These spectra contained rovibrational lines from water and OH. The water spectra were compared to simulations from the HITRAN and HITEMP databases. Furthermore, spectra were recorded in the flame of a flat methane/air burner and in an oxyacetylene flame produced by a welding torch. The results show that cavity enhanced absorption spectroscopy provides a sensitive method for rapid monitoring of species in radiative environments.

both techniques. The b 1 S g (v852) X 3 Sg (v950) transition of molecular oxygen around 628 nm is used to demonstrate the possibility to selectively measure either the polarization-dependent absorption or the resonant magneto-optical rotation of gas-phase molecules in the appropriate setup. Just as in CRD absorption spectroscopy, where the rate of absorption is measured, in the here presented polarization-dependent CRD ~PDCRD! detection scheme the rate of polarization rotation is measured, which enables the polarization rotation to be quantitatively determined. Apart from studying electro-optic and magneto-optic phenomena on gas-phase species, the PDCRD detection scheme is demonstrated to be applicable to the study of magneto-optical rotation in transparent solid samples as well. © 1997 American Institute of Physics.@S0021-9606~97!00836-2# In 1988 O’Keefe and Deacon demonstrated a new direct absorption spectroscopic technique that can be performed with a pulsed light source and that has a significantly higher sensitivity than obtainable in ‘‘conventional’’ absorption spectroscopy. This so-called cavity ring down ~CRD! technique is based upon the measurement of the rate of absorption rather than the magnitude of absorption of a light pulse confined in a closed optical cavity with a highQ factor. 1 The advantage over normal absorption spectroscopy results from ~i! the intrinsic insensitivity of the CRD technique to light source intensity fluctuations, and ~ii! the extremely long effective path lengths ~many kilometers! that can be realized in stable optical cavities. Since the technique is based on a pulsed measurement, it can be used in combination with pulsed molecular beams 2‐4 and it can also be used to study dynamical processes via time-resolved absorption measurements. 5 As long as mirrors with a sufficiently high reflectivity, detectors with a sufficiently fast time response, and tunable ~pulsed! light sources are available there is no intrinsic limitation to the spectral region in which CRD can be applied. By now, successful application of CRD spectroscopy has been demonstrated from the uv part of the spectrum 6 to the ir spectral region. 7,8 The application of CRD is most straightforward if it can be assumed that the linewidth of the light source can be neglected relative to the width of the molecular absorption. It has been shown, however, that if this assumption is no longer valid, one can still extract the correct absorption coefficient from the measured transients, provided the spectral intensity distribution of the light source is known. 6,9 As a consequence, it has been experimentally demonstrated that one does not necessarily need a narrow-band pulsed laser to perform a CRD experiment. One might just as well use a polychromatic light source and extract the spectral information after spectrally dispersing the light exiting the cavity, either in a monochromator or by using a Fourier transform spectrometer. 8,10

A compact open-path optical ammonia detector is developed. A tunable external-cavity diode laser operating at 1.5 μm is used to probe absorptions of ammonia via the cavity-enhanced absorption (CEA) technique. The detector is tested in a climate chamber. The sensitivity and linearity of this system are studied for ammonia and water at atmospheric pressure. A cluster of closely spaced rovibrational overtone and combination band transitions, observed as one broad absorption feature, is used for the detection of ammonia. On these molecular transitions a detection limit of 100 ppb (1 s) is determined. The ammonia measurements are calibrated independently with a chemiluminescence monitor. Compared to other optical open-path detection methods in the 1–2 μm region, the present result shows an improved sensitivity for contactless ammonia detection by over one order of magnitude. Using the same set-up, a detection limit of 100 ppm (1 s) is determined for the detection of water at atmospheric pressure.

The mid-infrared emission spectra of a large variety of astronomical objects are dominated by the aromatic infrared bands (AIBs), which are now widely accepted to originate from gaseous polycyclic aromatic hydrocarbons (PAHs). It is believed that the astrophysically most relevant molecules are at least 40–50 carbon atoms in size. Still, the large majority of laboratory experiments have been performed on smaller PAHs, mainly for reasons of experimental limitations and availability. Here, we show that combination of atmospheric pressure chemical ionization (APCI) with a direct insertion probe (DIP) inlet gives efficient access to larger, ionic PAHs for action spectroscopy studies. We present the gaseous IR spectra of two astrophysically relevant large PAHs, hexa-peri-hexabenzocoronene ( C 42 H 19 + ) and dicoronylene ( C 48 H 21 + ) in their protonated form. Compared to their radical cation analogs, the protonated species have a lower dissociation threshold as they can expel a neutral hydrogen radical leaving behind the resonance-stabilized radical cation; provided that the mass spectrometer can resolve precursor and product ions at one amu difference, this generates good quality spectra under multiple-photon dissociation conditions. Quantum-chemical computations at the density functional level are used to support experiments. Despite the apparent similarity of different protonation isomers, their IR spectra are predicted to be remarkably distinct. This facilitates a straightforward identification of the isomers formed experimentally. For both species studied, protonation occurs on the peripheral CH moiety in the ’bay region’ of the molecules. We compare the spectra of the protonated species with those of their radical cation analogs reported previously and briefly discuss the astrophysical relevance.