Apparatus for the study of electronic spectra of collisionally cooled cations: para-dichlorobenzene

Navigate Flying Molecular Elephants Safely to the Ground: Mass-Selective Soft Landing up to the Mega-Dalton Range by Electrospray Controlled Ion-Beam Deposition

The prototype of a highly versatile and efficient preparative mass spectrometry system used for the deposition of molecules in ultrahigh vacuum (UHV) is presented, along with encouraging performance data obtained using four model species that are thermolabile or not sublimable. The test panel comprises two small organic compounds, a small and very large protein, and a large DNA species covering a 4-log mass range up to 1.7 MDa as part of a broad spectrum of analyte species evaluated to date. Three designs of innovative ion guides, a novel digital mass-selective quadrupole (dQMF), and a standard electrospray ionization (ESI) source are combined to an integrated device, abbreviated electrospray controlled ion-beam deposition (ES-CIBD). Full control is achieved by (i) the square-wave-driven radiofrequency (RF) ion guides with steadily tunable frequencies, including a dQMF allowing for investigation, purification, and deposition of a virtually unlimited m/z range, (ii) the adjustable landing energy of ions down to ∼2 eV/z enabling integrity-preserving soft landing, (iii) the deposition in UHV with high ion beam intensity (up to 3 nA) limiting contaminations and deposition time, and (iv) direct coverage control via the deposited charge. The maximum resolution of R = 650 and overall efficiency up to T_total = 4.4% calculated from the solution to UHV deposition are advantageous, whereby the latter can be further enhanced by optimizing ionization performance. In the setup presented, a scanning tunneling microscope (STM) is attached for in situ UHV investigations of deposited species, demonstrating a selective, structure-preserving process and atomically clean layers.

An ion mobility mass spectrometer coupled with a cryogenic ion trap for recording electronic spectra of charged, isomer-selected clusters

Infrared and electronic spectra are indispensable for understanding the structural and energetic properties of charged molecules and clusters in the gas phase. However, the presence of isomers can potentially complicate the interpretation of spectra, even if the target molecules or clusters are mass-selected beforehand. Here, we describe an instrument for spectroscopically characterizing charged molecular clusters that have been selected according to both their isomeric form and their mass-to-charge ratio. Cluster ions generated by laser ablation of a solid sample are selected according to their collision cross sections with helium buffer gas using a drift tube ion mobility spectrometer and their mass-to-charge ratio using a quadrupole mass filter. The mobility- and mass-selected target ions are introduced into a cryogenically cooled, three-dimensional quadrupole ion trap where they are thermalized through inelastic collisions with an inert buffer gas (He or He/N₂ mixture). Spectra of the molecular ions are obtained by tagging them with inert atoms or molecules (Ne and N₂), which are dislodged following resonant excitation of an electronic transition, or by photodissociating the cluster itself following absorption of one or more photons. An electronic spectrum is generated by monitoring the charged photofragment yield as a function of wavelength. The capacity of the instrument is illustrated with the resonance-enhanced photodissociation action spectra of carbon clusters (C_n ⁺) and polyacetylene cations (HC_2nH⁺) that have been selected according to the mass-to-charge ratio and collision cross section with He buffer gas and of mass-selected Au₂ ⁺ and Au₂Ag⁺ clusters.

Demystifying the Diffuse Vibrational Spectrum of Aqueous Protons Through Cold Cluster Spectroscopy

The ease with which the pH is routinely determined for aqueous solutions masks the fact that the cationic product of Arrhenius acid dissolution, the hydrated proton, or H⁺(aq), is a remarkably complex species. Here, we review how results obtained over the past 30 years in the study of H⁺⋅(H₂O)_n cluster ions isolated in the gas phase shed light on the chemical nature of H⁺(aq). This effort has also revealed molecular-level aspects of the Grotthuss relay mechanism for positive-charge translocation in water. Recently developed methods involving cryogenic cooling in radiofrequency ion traps and the application of two-color, infrared-infrared (IR-IR) double-resonance spectroscopy have established a clear picture of how local hydrogen-bond topology drives the diverse spectral signatures of the excess proton. This information now enables a new generation of cluster studies designed to unravel the microscopic mechanics underlying the ultrafast relaxation dynamics displayed by H⁺(aq).

STARGATE: A new instrument for high-resolution photodissociation spectroscopy of cold ionic species

Spectroscopy of transient anions and radicals by gated and accelerated time-of-flight experiment is a new spectrometer developed in UCLouvain. This instrument measures high-resolution photodissociation spectra of mass-selected ions by the combination of a time-of-flight spectrometer including a specific gating, bunching, and re-referencing unit with a nanosecond pulsed dye laser, a pulsed deflection, and an energy selector. The ionic species are generated in a supersonic jet expansion by means of an electric discharge or by the impact of electrons coming from an electron gun. The versatility of the molecular systems that can be addressed by this instrument is illustrated by the presentation of mass spectra of cations, anions, and ionic clusters formed from different gas mixtures and backing pressures. The high-resolution spectrum of the A~²Σ⁺(002)←X~²Π_3/2(000) and A~²Σ⁺(002)←X~²Π_1/2(000) rovibronic bands of N₂O⁺ has been measured and analyzed to provide refined molecular parameters in the A~²Σ⁺(002) upper state. The A~²Σ⁺(002)←X~²Π_3/2(000) band has been used to evaluate the quality of the experimental setup in terms of rotational temperature, time of measurement for certain signal to noise ratio, and the accuracy of the determination of the wavenumber scale.

Introductory lecture: advances in ion spectroscopy: from astrophysics to biology

This introduction provides a historical context for the development of ion spectroscopy over the past half century by following the evolution of experimental methods to the present state-of-the-art. Rather than attempt a comprehensive review, we focus on how early work on small ions, carried out with fluorescence, direct absorption, and photoelectron spectroscopy, evolved into powerful technologies that can now address complex chemical problems ranging from catalysis to biophysics. One of these developments is the incorporation of cooling and temperature control to enable the general application of "messenger tagging" vibrational spectroscopy, first carried out using ionized supersonic jets and then with buffer gas cooling in radiofrequency ion traps. Some key advances in the application of time-resolved pump-probe techniques to follow ultrafast dynamics are also discussed, as are significant benchmarks in the refinement of ion mobility to allow spectroscopic investigation of large biopolymers with well-defined shapes. We close with a few remarks on challenges and opportunities to explore molecular level mechanics that drive macroscopic behavior.

Photoisomerization action spectrum of retinal protonated Schiff base in the gas phase

The photophysical behaviour of the isolated retinal protonated n-butylamine Schiff base (RPSB) is investigated in the gas phase using a combination of ion mobility spectrometry and laser spectroscopy. The RPSB cations are introduced by electrospray ionisation into an ion mobility mass spectrometer where they are exposed to tunable laser radiation in the region of the S1 ← S0 transition (420-680 nm range). Four peaks are observed in the arrival time distribution of the RPSB ions. On the basis of predicted collision cross sections with nitrogen gas, the dominant peak is assigned to the all-trans isomer, whereas the subsidiary peaks are assigned to various single, double and triple cis geometric isomers. RPSB ions that absorb laser radiation undergo photoisomerization, leading to a detectable change in their drift speed. By monitoring the photoisomer signal as a function of laser wavelength an action spectrum, extending from 480 to 660 nm with a clear peak at 615 ± 5 nm, is obtained. The photoisomerization action spectrum is related to the absorption spectrum of isolated retinal RPSB molecules and should help benchmark future electronic structure calculations.

An ion mobility mass spectrometer for investigating photoisomerization and photodissociation of molecular ions

An ion mobility mass spectrometry apparatus for investigating the photoisomerization and photodissociation of electrosprayed molecular ions in the gas phase is described. The device consists of a drift tube mobility spectrometer, with access for a laser beam that intercepts the drifting ion packet either coaxially or transversely, followed by a quadrupole mass filter. An ion gate halfway along the drift region allows the instrument to be used as a tandem ion mobility spectrometer, enabling mobility selection of ions prior to irradiation, with the photoisomer ions being separated over the second half of the drift tube. The utility of the device is illustrated with photoisomerization and photodissociation action spectra of carbocyanine molecular cations. The mobility resolution of the device for singly charged ions is typically 80 and it has a mass range of 100-440 Da, with the lower limit determined by the drive frequency for the ion funnels, and the upper limit by the quadrupole mass filter.

Electronic spectroscopy of carbon chains and rings of astrophysical interest

This perspective is concerned with laboratory measurements of the electronic spectra of carbon chains, rings, and their ions, including derivatives terminated by hydrogen and nitrogen atoms. The selected-species have relevance to astronomical observations through diffuse clouds, absorption features known as diffuse interstellar bands (DIBs). Two indications to decide which molecules should be studied are the observations of polar carbon chains in dense clouds by rotational spectroscopy and the knowledge that a certain number of these have electronic transitions in the DIB region. This information has been obtained initially by measurements of the electronic absorptions in 6 K neon matrixes using mass-selection. This was followed by the gas-phase observations using cavity ringdown and resonance enhanced techniques in combination with pulsed-supersonic discharge sources or via laser vaporization. The gas-phase spectra were then compared with DIB data, all with negative results, except for the detection of C3, but leading to upper limits of their column densities <10(12) cm–2. By reference to mm-wave absorption measurements in the diffuse medium, it is shown that, although species such as H2C3 are present there, the product of the expected column densities and oscillator strength of the transitions will lead to only very weak DIBs. The significant conclusion is that carbon chains and their derivatives containing hydrogen or nitrogen comprising up to a dozen atoms cannot be responsible for stronger DIBs. However, chains with an odd-number of carbon atoms, C17, C19, ···, have very intense transitions in the region above 4400 Å and remain attractive candidates. An uncertainty is the excited electronic state lifetime; if this is less than 70 fs, then the resulting absorptions would be too broad to be astronomically relevant. The electronic absorptions of some of the species studied bear a striking resemblance to DIB data. The two peaked rotational contour of the origin band in the electronic transition of dicyanoacetylene cation is superimposable on a DIB absorption when shifted by 1 Å. The band profiles of cyclic C18 at 100 or 20 K are similar to DIBs but differ in wavelength. This suggests that another set of potential candidates are the carbon rings of sizes up to a hundred of atoms, including ions and heavy atoms, with the requirement of a large oscillator strength. Observations on the absorptions of propadienylidene C3H2 and C60+ are discussed.

Note: New design of a cryogenic linear radio frequency multipole trap

A new design of a cryogenic linear 22-pole ion trap has been constructed and tested. It is essentially a copper housing to which opposite inner walls two electrode sets are attached via sapphire insulators. These stainless steel electrodes are electroformed in one piece to guarantee good heat conduction. Connected to an external coil, they form an LC-circuit of about 19 MHz resonance frequency. This circuit is excited with a rf power supply made of a commercial digital synthesizer followed by a 10 W amplifier. Buffer gas-cooled H(2)D(+) ions have been stored in this trap at a nominal trap temperature of 14 K. Spectroscopy of the ions confirmed that the kinetic (Doppler) temperature is in reasonable agreement with this value.

Electronic Spectroscopy of Carbon Chains

Investigators have recorded the electronic spectra of assorted carbon-chain systems in the gas phase using a variety of methods, ranging from direct cavity ringdown absorption spectroscopy to photofragmentation techniques that utilize the cooling capabilities of an ion trap. We summarize the results from these studies and compare them with astronomical measurements of the diffuse interstellar band (DIB) absorptions. Although carbon chains comprising up to a handful of carbon atoms cannot be the carrier species, we explore which chains remain viable. In particular, the 1Σu+–X1Σg+ transitions of the odd-numbered carbon chains (n = 17,19,…) possess large oscillator strengths and lie in the 400–900-nm DIB range. The origin bands of larger bare carbon rings, such as C18, have also been observed, with striking similarities to some DIB measurements at high resolution, although at other wavelengths. Finally, we consider recently obtained electronic spectra of metal-containing carbon chains.

Rotational state selection of a CH3I+ ion beam using vacuum ultraviolet-mass-analyzed threshold ionization spectroscopy: characterization using photodissociation spectroscopy

The A(2)A(1)<--X(2)E(3/2) transition of CH(3)I(+) was investigated by photodissociation (PD) of the cation generated by one-photon mass-analyzed threshold ionization (MATI). Compared to the PD spectrum obtained by excitation of the cation in the main 0-0 band in the MATI spectrum, those obtained by excitation of the cations in the satellite structures showed substantially simplified rotational structures for nondegenerate vibronic bands. Spectral simplification occurred because each satellite consisted mostly of cations with one K quantum number. Spectroscopic constants in the ground vibronic state and in the 2(1)3(5), 2(1)3(8), 3(9), and 3(13) nondegenerate vibrational states in A(2)A(1) were determined via spectral fitting. Also, those in the 2(1)3(n)6(1) (n=1?) degenerate state, which had been reported previously, was improved. The K quantum number in each satellite determined by the present high resolution study was compatible with the prediction by the symmetry selection rule for photoionization. That is, the K quantum number of the ion core in high Rydberg states accessed by one-photon excitation was found to be conserved upon pulsed field ionization. This work demonstrates generation of mass-selected, vibronically selected, and K-selected ion beam by one-photon MATI.

High resolution spectroscopy for the A (2)A(1) state of CH(3)I(+) by mass-analyzed threshold ionization/photodissociation

Photodissociation of CH(3)I(+) in the ground vibronic state generated by mass-analyzed threshold ionization resulted in a superb spectrum for the first excited electronic state (A (2)A(1)) with hardly any spurious peak. Rotational structure in the spectrum could be resolved by using a single mode laser. This structure for one vibronic band, 2(1)3(1)6(1), was analyzed with the assumption of Hund's case (a) scheme both in the ground and excited electronic states.

Electronic gas-phase spectra of larger polyacetylene cations

The origin bands of the A 2Pi-X 2Pi electronic transition for three new linear polyacetylene cation chains, HC12H+, HC14H+, and HC16H+, have been recorded in the gas phase at approximately 30 K, located at 924.7, 1034.6, and 1144.0 nm. The absorption spectra were observed using a two-color two-photon ion-photodissociation experiment that utilizes the cooling capabilities of a 22-pole ion trap. Such spectra allow a direct comparison between laboratory and astrophysical data; however, no matches were found between the experimentally determined origin bands and the known diffuse interstellar bands.

Electronic absorption spectra of the protonated polyacetylenes HC2nH2+ (n=3,4) in the gas phase

A new approach has been developed for the purpose of measuring the electronic transitions to bound exited states for cations that have been collisionally relaxed to low vibrational and rotational temperatures. This has been used to obtain the first gas phase electronic spectra of the protonated polyacetylenes using a two-color ion-photodissociation approach. Specifically, the origin bands in the B (1)A(1)<-- X(1)A(1) transitions of HC(6)H(2) (+) and HC(8)H(2) (+) (C(2v) geometry) were observed at 26,403.3 and 21,399.8 cm(-1). Data on such cooled systems allow a direct comparison between laboratory and astrophysical measurements.