Pulsed Field Ionization Research Articles

Astronomical CH3+ rovibrational assignments

Context. The methyl cation (CH3+) has recently been discovered in the interstellar medium through the detection of 7 μm (1400 cm−1) features toward the d203-506 protoplanetary disk by the JWST. Line-by-line spectroscopic assignments of these features, however, were unsuccessful due to complex intramolecular perturbations preventing a determination of the excitation and abundance of the species in that source. Aims. Comprehensive rovibrational assignments guided by theoretical and experimental laboratory techniques provide insight into the excitation mechanisms and chemistry of CH3+ in d203-506. Methods. The rovibrational structure of CH3+ was studied theoretically by a combination of coupled-cluster electronic structure theory and (quasi-)variational nuclear motion calculations. Two experimental techniques were used to confirm the rovibrational structure of CH3+:(1) infrared leak-out spectroscopy of the methyl cation, and (2) rotationally resolved photoelectron spectroscopy of the methyl radical (CH3). In (1), CH3+ ions, produced by the electron impact dissociative ionization of methane, were injected into a 22-pole ion trap where they were probed by the pulses of infrared radiation from the FELIX free electron laser. In (2), neutral CH3, produced by CH3NO2 pyrolysis in a molecular beam, was probed by pulsed-field ionization zero-kinetic-energy photoelectron spectroscopy. Results. The quantum chemical calculations performed in this study have enabled a comprehensive spectroscopic assignment of the v2+ and v4+ bands of CH3+ detected by the JWST. The resulting spectroscopic constants and derived Einstein A coefficients fully reproduce both the infrared and photoelectron spectra and permit the rotational temperature of CH3+ (T = 660 ± 80 K) in d203-506 to be derived. A beam-averaged column density of CH3+ in this protoplanetary disk is also estimated.

Effects of rotational excitation on decay rates of long-lived Rydberg states in NO.

Nitric oxide (NO) molecules in pulsed supersonic beams have been excited to long-lived Rydberg-Stark states in series converging to the lowest vibrational level in the ground electronic state of NO+ with rotational quantum numbers N+ = 2, 4, and 6. The molecules in these excited states were then guided, or decelerated and trapped in a chip-based Rydberg-Stark decelerator, and detected in situ by pulsed electric field ionization. Time constants, reflecting the decay of molecules in N+ = 2 Rydberg-Stark states, with principal quantum numbers n between 38 and 44, from the electrostatic traps were measured to be ∼300μs. Molecules in Rydberg-Stark states with N+ = 4 and 6, and the same range of values of n were too short-lived to be trapped, but their decay time constants could be determined from complementary sets of delayed pulsed electric field ionization measurements to be ∼100 and ∼25 μs, respectively.

Precise determination of adiabatic ionisation energies of large polyatomic molecules: p-diaminobenzene

We present a procedure, based on established principles, to accurately determine the ionisation energies of medium-sized to large polyatomic molecules in supersonic beams. The method combines single-photon excitation from the electronic ground state using tunable vacuum-ultraviolet laser radiation, the delayed pulsed-field ionisation of high Rydberg states with sequences of pulsed electric fields, and the modelling of the lineshapes from calculations of the field-ionisation dynamics of high Rydberg states and of the rotational contours of the photoelectron bands. The method is illustrated by a measurement of the adiabatic ionisation energy of p-diaminobenzene (p-DB), for which we obtain the value of 54,617.47(15) cm , which represents a more than 20-fold accuracy improvement over previous determinations.

Role of Coulomb antiblockade in the photoassociation of long-range Rydberg molecules

We present a new mechanism contributing to the detection of photoassociated long-range Rydberg molecules via pulsed-field ionization: ionic products, created by the decay of a long-range Rydberg molecule, modify the excitation spectrum of surrounding ground-state atoms and facilitate the excitation of further atoms into Rydberg states by the photoassociation light. Such an ion-mediated excitation mechanism has been previously called "Coulomb anti-blockade". Pulsed-field ionisation typically doesn't discriminate between the ionization of a long-range Rydberg molecule and an isolated Rydberg atom, and thus the number of atomic ions detected by this mechanism is not proportional to the number of long-range Rydberg molecules present in the probe volume. By combining high-resolution UV and RF spectroscopy of a dense, ultracold gas of cesium atoms, theoretical modeling of the molecular level structures of long-range Rydberg molecules bound below nP_3/2 Rydberg states of cesium, and a rate model of the photoassociation and decay processes, we unambiguously identify the signatures of this detection mechanism in the photoassociation of long-range Rydberg molecules bound below atomic asymptotes with negative Stark shifts.

Electrostatic trapping vibrationally excited Rydberg NO molecules

Nitric oxide (NO) molecules travelling in pulsed supersonic beams have been laser photoexcited to long-lived Rydberg states in series converging to the lowest lying, , and first excited, , vibrational states in the ground electronic state of NO. After excitation, the molecules were decelerated in the travelling electric traps of a chip-based Rydberg–Stark decelerator. Deceleration to rest in the laboratory-fixed frame of reference allowed subsequent electrostatic trapping before in situ detection by pulsed electric field ionisation. The decay rates of the molecules from the traps were measured for states with principal quantum numbers between 32 and 48. Comparison of the corresponding trap decay time constants, ranging from 245 to s for states with , with those recorded for molecules in states with , allowed bounds to be placed on the vibrational autoionisation rates of the ℓ-mixed Stark states prepared in the experiments of Hz.

Cold ion chemistry within a Rydberg-electron orbit: test of the spectator role of the Rydberg electron in the He(n) + CO → C(n′) + O + He reaction

Recently, a new method has been introduced to study ion-molecule reactions at very low collision energies, down to below k B ⋅ 1 K (Allmendinger et al 2016 ChemPhysChem 17 3596). To eliminate the acceleration of the ions by stray electric fields in the reaction volume, the reactions are observed within the orbit of a Rydberg electron with large principal quantum number n > 20. This electron is assumed not to influence the reaction taking place between the ion core and the neutral molecules. This assumption is tested here with the example of the He(n) + CO → C(n′) + O + He reaction, which is expected to be equivalent to the He+ + CO → C+ + O + He reaction, using a merged-beam approach enabling measurements of relative reaction rates for collision energies E coll in the range from 0 to about k B ⋅ 25 K with a collision-energy resolution of ∼k B ⋅ 200 mK at E coll = 0. In contrast to the other ion-molecule reactions studied so far with this method, the atomic ion product (C+) is in its electronic ground state and does not have rotational and vibrational degrees of freedom so that the corresponding Rydberg product [C(n′)] cannot decay by autoionization. Consequently, one can investigate whether the principal quantum number is effectively conserved, as would be expected in the spectator Rydberg-electron model. We measure the distribution of principal quantum numbers of the reactant He(n) and product C(n′) Rydberg atoms by pulsed-field ionization following initial preparation of He(n) in states with n values between 30 and 45 and observe that the principal quantum number of the Rydberg electron is conserved during the reaction. This observation indicates that the Rydberg electron is not affected by the reaction, from which we can conclude that it does not affect the reaction either. This conclusion is strengthened by measurements of the collision-energy-dependent reaction yields at n = 30, 35 and 40, which exhibit the same behavior, i.e. a marked decrease below E coll ≈ k B ⋅ 5 K.

Phenotypic and Genetic Comparison of a Plant-Internalized and an Animal-Isolated Salmonella Choleraesuis Strain.

Contamination of fresh produce with human pathogens poses an important risk for consumers, especially after raw consumption. Moreover, if microorganisms are internalized, no removal by means of further hygienic measures would be possible. Human pathogenic bacteria identified in these food items are mostly of human or animal origin and an adaptation to this new niche and particularly for internalization would be presumed. This study compares a plant-internalized and an animal-borne Salmonella enterica subsp. enterica serovar Choleraesuis aiming at the identification of adaptation of the plant-internalized strain to its original environment. For this purpose, a phenotypical characterization by means of growth curves under conditions resembling the indigenous environment from the plant-internalized strain and further analyses using Pulsed-field gel electrophoresis and Matrix-assisted laser desorption ionization time of flight spectrometry were assessed. Furthermore, comparative genomic analyses by means of single nucleotide polymorphisms and identification of present/absent genes were performed. Although some phenotypical and genetic differences could be found, no signs of a specific adaptation for colonization and internalization in plants could be clearly identified. This could suggest that any Salmonella strain could directly settle in this niche without any evolutionary process being necessary. Further comparative analysis including internalized strains would be necessary to assess this question. However, these kinds of strains are not easily available.

Line shapes and line positions in PFI-ZEKE photoelectron and MATI spectra of positively charged ions

A model developed to calculate the positions and shapes of the lines observed in pulsed-field-ionisation zero-kinetic-energy photoelectron (PFI-ZEKE-PE) and mass-analysed threshold ionisation (MATI) spectra of neutral molecules [Hollenstein et al., J. Chem. Phys. 115, 5461 (2001)] is generalised to positively charged systems of arbitrary charge. The model assumes that the pulsed field ionisation of the high Rydberg states detected experimentally is diabatic and that their ionisation rates in the presence of electric fields can be calculated with the asymptotic expressions of Damburg and Kolosov [J. Phys. B 12, 2637 (1979)]. We use this model to calculate the shifts of the lines with respect to the field-free ionisation thresholds and their widths and find that, to an excellent approximation, both the line shifts and line widths scale with . We also derive simple analytical expressions that can be used to estimate the line shifts and line widths for typical electric-field pulse sequences used in PFI-ZEKE-PE and MATI spectroscopy of positively charged cations.

Soft X-ray Induced Production of Neutral Fragments in High-Rydberg States at the O 1s Ionization Threshold of the Water Molecule.

Dissociation of water molecules after soft X-ray absorption can yield neutral fragments in high-Rydberg (HR) states. We have studied the production of such fragments by field ionization and ion time-of-flight (TOF) spectrometry. Neutral HR fragments are created at all resonances below the O 1s ionization potential (IP) and particularly within 1 eV above the O 1s IP. The latter effect is due to the recapture of the O 1s photoelectrons into HR orbitals of the molecular water ion after the emission of a fast Auger electron. H2O+(HR) fragments subsequently dissociate, yielding neutral H(HR) and O(HR) fragments, as were found by measuring the TOF spectra by pulsed field ionization. Such measurements were carried out at the O 1s → 4a1 and 2b2 resonances as well as just above the O 1s IP. The TOF spectra also reveal two series of oscillatory structures that are attributed to quantum beats involving Lyman emission in one of the series and field ionization of H(HR) fragments in the other series.

Accurate determination of ionization energy of 1, 3-diethoxybenzene via photoionization efficiency spectrum in electrostatic field

Ionization energy (IE) is an important characteristic parameter of atoms or molecules. It plays an important role in the process of photophysics and photochemistry. The precise ionization energy is very important for relevant research. Especially, it is very useful for adjusting the signal of the zero-kinetic energy (ZEKE) spectrum, and it also plays a key role in judging the number of rotamers and molecular configuration. In linear time-of-flight mass spectrometers, pulsed electric fields are usually used to drive photo-ionized ions to the detector to produce the photoionization efficiency (PIE) spectrum. The ionization energy is directly obtained from the PIE curve. The uncertainty of the measured IE is usually greater than or equal to ± 10 cm<sup>–1</sup>. The ZEKE spectroscopy is based on the long-lived Rydberg state field ionization technology. In the ZEKE experiments, the laser excites molecules to the Rydberg state and then a pulsed field ionization (PFI) is used for measurement. A peak with high signal-to-noise ratio and narrow linewidth signal appears near the ionization threshold. Therefore, the more accurate ionization energy can be obtained, and the uncertainty of the measured value is about ± 5 cm<sup>–1</sup>. The 1,3-diethoxybenzene is an important benzene derivative, and experiments have confirmed that there are two rotamers, i.e. I (down-up) and III (down-down) in the supersonic molecular beam. In this paper, a linear time-of-flight mass spectrometer is used to measure the photoionization efficiency curves of 1,3-diethoxybenzene in electrostatic fields. From the linear fitting of the ionization energy values measured under different electric fields (Stark effect) to the square root of the field strengths, the precise ionization energy values of rotamer I and rotamer III are determined to be (62419 ± 2) cm<sup>–1</sup> and (63378 ± 2) cm<sup>–1</sup>, respectively. Compared with the accuracies of the values measured by the usual pulsed electric field acceleration mechanism and the ZEKE spectroscopy, the accuracy is improved from about ± 10 and ± 5 to ± 2 cm<sup>–1</sup>, respectively. The physical mechanism, advantages and disadvantages of different methods are analyzed and discussed. The present research results show that the ionization energy measured in the electrostatic field is more accurate, the physical meaning of the measurement process is clear, and the threshold data are easy to collect. This is the first report on the precise ionization energy of 1,3-diethoxybenzene rotamers.

Cavity-enhanced Ramsey spectroscopy at a Rydberg-atom–superconducting-circuit interface

The coherent interaction of Rydberg helium atoms with microwave fields in a λ/4 superconducting coplanar waveguide resonator has been exploited to probe the spectral characteristics of an individual resonator mode. This was achieved by preparing the atoms in the 1s55s 3S1 Rydberg level by resonance enhanced two-color two-photon excitation from the metastable 1s2s 3S1 level. The atoms then traveled over the resonator in which the third harmonic microwave field, at a frequency of ωres=2π×19.556 GHz, drove the two-photon 1s55s 3S1 → 1s56s 3S1 transition. By injecting a sequence of Ramsey pulses into the resonator and monitoring the coherent evolution of the Rydberg state population by state-selective pulsed electric field ionization as the frequency of the microwave field was tuned, spectra were recorded that allowed the resonator resonance frequency and quality factor to be determined with the atoms acting as microscopic quantum sensors.

Slow Decay Processes of Electrostatically Trapped Rydberg NO Molecules.

Nitric oxide (NO) molecules initially traveling at 795 m/s in pulsed supersonic beams have been photoexcited to long-lived hydrogenic Rydberg-Stark states, decelerated and electrostatically trapped in a cryogenically cooled, chip-based transmission-line Rydberg-Stark decelerator. The decelerated and trapped molecules were detected insitu by pulsed electric field ionization. The operation of the decelerator was validated by comparison of the experimental data with the results of numerical calculations of particle trajectories. Studies of the decay of the trapped molecules on timescales up to 1ms provide new insights into the lifetimes of, and effects of blackbody radiation on, Rydberg states of NO.

Coupling Rydberg Atoms to Microwave Fields in a Superconducting Coplanar Waveguide Resonator.

Rydberg helium atoms traveling in pulsed supersonic beams have been coupled to microwave fields in a superconducting coplanar waveguide (CPW) resonator. The atoms were initially prepared in the 1s55s ^{3}S_{1} Rydberg level by two-color two-photon laser excitation from the metastable 1s2s ^{3}S_{1} level. Two-photon microwave transitions between the 1s55s ^{3}S_{1} and 1s56s ^{3}S_{1} levels were then driven by the 19.556GHz third-harmonic microwave field in a quarter-wave CPW resonator. This superconducting microwave resonator was fabricated from niobium nitride on a silicon substrate and operated at temperatures between 3.65 and 4.30K. The populations of the Rydberg levels in the experiments were determined by state-selective pulsed electric field ionization. The coherence of the atom-resonator coupling was studied by time-domain measurements of Rabi oscillations.

Excitation and characterization of long-lived hydrogenic Rydberg states of nitric oxide.

High Rydberg states of nitric oxide (NO) with principal quantum numbers between 40 and 100 and lifetimes in excess of 10 µs have been prepared by resonance enhanced two-color two-photon laser excitation from the X 2Π1/2 ground state through the A 2Σ+ intermediate state. Molecules in these long-lived Rydberg states were detected and characterized 126 µs after laser photoexcitation by state-selective pulsed electric field ionization. The laser excitation and electric field ionization data were combined to construct two-dimensional spectral maps. These maps were used to identify the rotational states of the NO+ ion core to which the observed series of long-lived hydrogenic Rydberg states converge. The results presented pave the way for Rydberg-Stark deceleration and electrostatic trapping experiments with NO, which are expected to shed further light on the decay dynamics of these long-lived excited states, and are of interest for studies of ion-molecule reactions at low temperatures.

Finite size cavity effect on nS rubidium Rydberg state lifetimes

In this work, we present lifetime measurements of nS states of Rb as a function of the principal quantum number (40 n 70) using a sample of cold atoms held in a magneto-optical trap, which is performed in a finite size metal vacuum chamber. The Rydberg states are excited through a two-photon transition, and detected by pulsed field ionization. Our measurements are larger than the predictions by well established theoretical model (1984 Phys. Rev. A 30 2881 and 2009 Phys. Rev. A 79 052504). We have implemented a theoretical model, which considers the vacuum chamber as a lossy Fabry–Perot cavity with a discrete spectrum, and compared with experimental results. Such comparison indicates that the blackbody radiation contribution on Rydberg state lifetime can be decreased by using a small size metal cavity, without the need of cryogenic environment. This effect may have application in experiments where longer Rydberg lifetimes are required.

Fluorescence-lifetime-limited trapping of Rydberg helium atoms on a chip

Metastable (1s)(2s) helium atoms produced in a supersonic beam were excited to Rydberg-Stark states (with n in the 27–30 range) in a cryogenic environment and subsequently decelerated by, and trapped above, a surface-electrode decelerator. In the trapping experiments, the Rydberg atoms were brought to rest in 75 μs and over a distance of 33 mm and kept stationary for times in the 0–525 μs range, before being re-accelerated for detection by pulsed field ionisation. The use of a home-built valve producing short gas pulses with a duration of about 20 μs enabled the reduction of losses arising from collisions with atoms in the trailing part of the gas pulses. Cooling the decelerator to 4.7 K further suppressed losses by transitions induced by blackbody radiation and by collisions with atoms desorbing from the decelerator surface. The main contribution (60%) to the atom loss during deceleration is attributed to the escape out of the decelerator moving traps of atoms having energies higher than the trap saddle point, spontaneous emission and collisions with atoms in the trailing part of the gas pulses causing each only about 20% of the atom loss. At 4.7 K, the atom losses in the trapping phase of the experiments were found to be almost exclusively caused by spontaneous emission and the trap lifetimes were found to correspond to the natural lifetimes of the Rydberg-Stark states. Increasing the temperature to 100 K enhanced the trap losses by transitions stimulated by blackbody radiation.

Rydberg-state ionization dynamics and tunnel ionization rates in strong electric fields

Tunnel ionization rates of triplet Rydberg states in helium with principal quantum numbers close to 37 have been measured in electric fields at the classical ionization threshold of $\sim197$ V/cm. The measurements were performed in the time domain by combining high-resolution continuous-wave laser photoexcitation and pulsed electric field ionization. The observed tunnel ionization rates range from $10^5$ s$^{-1}$ to $10^7$ s$^{-1}$ and have, together with the measured atomic energy-level structure in the corresponding electric fields, been compared to the results of calculations of the eigenvalues of the Hamiltonian matrix describing the atoms in the presence of the fields to which complex absorbing potentials have been introduced. The comparison of the measured tunnel ionization rates with the results of these, and additional calculations for hydrogen-like Rydberg states performed using semi-empirical methods, have allowed the accuracy of these methods of calculation to be tested. For the particular eigenstates studied the measured ionization rates are $\sim~5$ times larger than those obtained from semi-empirical expressions.

PFI-ZEKE photoelectron spectroscopy of positively charged ions: illustration with Mg+

Pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy has been applied for the first time to a positively charged ion, Mg+. Mg+ ions have been prepared in the [Ne]3s S21/2 ground state by photoionization of Mg atoms in a supersonic expansion. The ions were then excited to very high Rydberg states located just below the ionization threshold corresponding to the formation of ground-state Mg2+ by (2 + 1′) resonance-enhanced three-photon absorption via the 3d D25/2 intermediate state. We demonstrate experimentally and in a calculation of the transition moments that only the nf Rydberg series of Mg+ is efficiently excited from the D25/2 state and extract from the Rydberg spectrum precise values for the ionization energy of Mg+ (121267.65 ± 0.06 cm−1) and the quantum defect of the nf series (0.0044(6)). The field ionization of very high Rydberg states of Mg+ was investigated using sequences of electric field pulses. Compared to the pulsed field ionization of neutral atoms or molecules, the pulsed field ionization of positively charged ions differs in three respects: (i) the shift of the ionization thresholds induced by an electric field of given strength is larger by a factor of approximately 2, (ii) the inherent presence of ions in the photoexcitation volume prevents the escape of slow electrons, and (iii) the inhomogeneous stray fields induced by these ions and their effects on the high Rydberg states reduce the selectivity of the field-ionization process and degrade the resolution of the PFI-ZEKE photoelectron spectra. By minimizing the ion density, optimizing the photoexcitation efficiency and using multipulse field-ionization sequences, we show that the resolution of the photoelectron spectra can be as high as 0.3 cm−1, which opens up the possibility to study doubly-charged cations by high-resolution photoelectron spectroscopy.

Laser photoexcitation of Rydberg states in helium with n > 400

Helium atoms travelling in pulsed supersonic beams have been photoexcited from the metastable 1s2s 3S1 level to Rydberg states with principal quantum numbers exceeding 400 by resonance-enhanced two-colour two-photon excitation using narrow-bandwidth CW laser radiation. To achieve this, the photoexcitation region was shielded and stray electric fields cancelled using a cubic arrangement of 80 mm × 80 mm copper plates. The excited Rydberg atoms were detected by pulsed electric field ionisation downstream from this photoexcitation region. Comparison of the experimental spectra with the results of calculations indicate that the stray electric fields at the position of Rydberg state photoexcitation were reduced to as low as 160 μV cm–1. Using helium in these experiments minimises temporal changes in the stray fields resulting from surface adsorption. Measurements performed 70 h apart indicate that the rate of change of these fields was on the order of 2 μV cm–1 per h, making beams of metastable helium atoms ideally suited as probes for high precision electrometry in spectroscopy experiments involving less readily available species such as positronium or antihydrogen.

High-resolution photoelectron spectroscopy and calculations of the highest bound levels of below the first dissociation threshold

Pulsed-field ionization zero-kinetic-energy photoelectron spectra of D2 have been recorded from the intermediate state to determine the positions of bound rovibronic levels of located within 1400 cm−1 of the D+ + D(1s) dissociation threshold. The ion-pair character of the intermediate state resulted in large changes of the rotational quantum number upon photoionization, which enabled the observation of levels of with rotational quantum number as high as 10. The experimental data cover a range of levels within which the usual hierarchy of timescales of the electronic, vibrational and rotational motions is inverted. The term values of these levels with respect to the X rovibronic ground state of D2 and the energy intervals of the ionic states, measured with an accuracy of typically 0.11 cm−1 and 0.02 cm−1, respectively, are compared with positions calculated ab initio at various degrees of approximation, starting from the Born–Oppenheimer approximation and successively including adiabatic, nonadiabatic, relativistic and radiative corrections. The comparison shows that the accuracy of the photoelectron-spectroscopic measurement is sufficient to reveal the effects of the adiabatic, nonadiabatic, relativistic and radiative corrections on the absolute term values. Comparing our calculations, which rely on an approximate evaluation of the nonadiabatic corrections based on effective R-dependent reduced masses, with the theoretical results for by Moss (1993 J. Chem. Soc. Faraday Trans. 89 3851) and for by Wolniewicz and Orlikowski (1991 Mol. Phys. 74 103) enables the quantification of the errors introduced by our approximative treatment of the nonadiabatic corrections. Improved rotational term values of the level were also derived.