Gas-phase infrared multiple photon dissociation spectroscopy of mass-selected molecular ions (original) (raw)
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Laboratory Infrared Spectroscopy of Cationic Polycyclic Aromatic Hydrocarbon Molecules
The Astrophysical Journal, 2003
Infrared spectroscopy of a variety of interstellar sources shows strong mid-IR emission bands, which are generally attributed to emission from highly vibrationally excited polycyclic aromatic hydrocarbon molecules (PAHs) in the neutral and, particularly, cationic states. Over the past decade, various experimental methods have been developed to record the infrared spectra of cationic PAHs in the laboratory. In this paper, we discuss available experimental spectra obtained with matrix isolation spectroscopy (MIS), infrared multiple-photon dissociation of trapped ions (MPD), dissociation spectroscopy of ionic PAH van der Waals clusters (VDW), and infrared emission (IRE). Moreover, we compare these experimental spectra to density functional theory (DFT) calculations. The main body of experimental data relies on MIS and MPD spectra, and we present a detailed comparison of results from these methods, providing linear and multiple-photon absorption data, respectively. The effects of multiple-photon absorption, as encountered in MPD, and multiple-photon emission, occurring in interstellar spectra, are carefully assessed with the use of mathematical models, which include the effects of vibrational anharmonicity. We show that an analysis of the multiple-photon and linear data can provide important information on the anharmonicity parameters, which is otherwise difficult to attain. This is illustrated with a detailed comparison of the linear and multiplephoton absorption spectra of the naphthalene cation, yielding experimental anharmonicity parameters for the IR-active modes in the 500-1700 cm À1 range.
Review of Scientific Instruments, 2005
A Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer has been installed at a free electron laser (FEL) facility to obtain infrared absorption spectra of gas phase ions by infrared multiple photon dissociation (IRMPD). The FEL provides continuously tunable infrared radiation over a broad range of the infrared spectrum, and the FT-ICR mass spectrometer, utilizing a 4.7 Tesla superconducting magnet, permits facile formation, isolation, trapping, and high-mass resolution detection of a wide range of ion classes. A description of the instrumentation and experimental parameters for these experiments is presented along with preliminary IRMPD spectra of the singly-charged chromium-bound dimer of diethyl ether ͑Cr͑C 4 H 10 O͒ 2 + ͒ and the fluorene molecular ion ͑C 13 H 10 + ͒. Also presented is a brief comparison of the fluorene cation spectrum obtained by the FT-ICR-FEL with an earlier spectrum recorded using a quadrupole ion trap (QIT).
Physical Review A
The photoprocessing of astrophysically relevant small polycyclic aromatic hydrocarbon (PAH) molecules, namely, anthracene, phenanthrene, and pentacene, under intense UV field is experimentally studied to explore the formation of smaller ions upon their dissociative ionization and the effect of their size and structure on the dynamics of dissociative ionization. The molecules are UV processed in the multiphoton regime using a 266-nm nanosecond laser pulse. The dissociative ionization spectra at different laser intensities reveal distinct dissociation dynamics of these PAHs based on their size and structures.
INFRARED SPECTRA OF ISOLATED PROTONATED POLYCYCLIC AROMATIC HYDROCARBON MOLECULES
The Astrophysical Journal, 2009
Gas-phase infrared (IR) spectra of larger protonated polycyclic aromatic hydrocarbon molecules, H + PAH, have been recorded for the first time. The ions are generated by electrospray ionization and spectroscopically assayed by IR multiple-photon dissociation (IRMPD) spectroscopy in a Fourier transform ion cyclotron resonance mass spectrometer using a free electron laser. IRMPD spectra of protonated anthracene, tetracene, pentacene, and coronene are presented and compared to calculated IR spectra. Comparison of the laboratory IR spectra to an astronomical spectrum of the unidentified IR emission (UIR) bands obtained in a highly ionized region of the interstellar medium provides for the first time compelling spectroscopic support for the recent hypothesis that H + PAHs contribute as carriers of the UIR bands.
The Journal of Chemical Physics, 2010
C 3 H 3 + ions produced with a pulsed discharge source and cooled in a supersonic beam are studied with infrared laser photodissociation spectroscopy in the 800-4000 cm −1 region using the rare gas tagging method. Vibrational bands in the C-H stretching and fingerprint regions confirm the presence of both the cyclopropenyl and propargyl cations. Because there is a high barrier separating these two structures, they are presumed to be produced by different routes in the plasma chemistry; their relative abundance can be adjusted by varying the ion source conditions. Prominent features for the cyclopropenyl species include the asymmetric carbon stretch ͑ 5 ͒ at 1293 cm −1 and the asymmetric C-H stretch ͑ 4 ͒ at 3182 cm −1 , whereas propargyl has the CH 2 scissors ͑ 4 ͒ at 1445, the CC triple bond stretch ͑ 3 ͒ at 2077 and three C-H stretches ͑ 2 , 9 , and 1 ͒ at 3004, 3093, and 3238 cm −1. Density functional theory computations of vibrational spectra for the two isomeric ions with and without the argon tag reproduce the experimental features qualitatively; according to theory the tag atom only perturbs the spectra slightly. Although these data confirm the accepted structural pictures of the cyclopropenyl and propargyl cations, close agreement between theoretical predictions and the measured vibrational band positions and intensities cannot be obtained. Band intensities are influenced by the energy dependence and dynamics of photodissociation, but there appear to be fundamental problems in computed band positions independent of the level of theory employed. These new data provide infrared signatures in the fingerprint region for these prototypical carbocations that may aid in their astrophysical detection.
Gas-phase IR−UV double-resonance laser spectroscopy is an IR absorption technique that bridges the gap between experimental IR spectroscopy and theory. The IR experiments are used to directly evaluate predicted frequencies and potential energy surfaces as well as to probe the structure of isolated molecules. However, a detailed understanding of the underlying mechanisms is, especially in the far-IR regime, still far from complete, even though this is crucial for properly interpreting the recorded IR absorption spectra. Here, events occurring upon excitation to vibrational levels of polycyclic aromatic hydrocarbons by far-IR radiation from the FELIX free electron laser are followed using resonance-enhanced multiphoton ionization spectroscopy. These studies provide detailed insight into how ladder climbing and anharmonicity influence IR−UV spectroscopy and therefore the resulting IR signatures in the far-IR region. Moreover, the potential energy surfaces of these low-frequency delocalized modes are investigated and shown to have a strong harmonic character.
Rapid Communications in Mass Spectrometry, 2019
Rationale: Methods for isomer discrimination by mass spectroscopy are of increasing interest. Here we describe the development of a 3D ion trap for Infrared Multiple Photon Dissociation (IRMPD) spectroscopy that enables the acquisition of the infrared spectrum of selected ions in the gas phase. This system is suitable for the study of myriad chemical systems, including isomer mixtures. Methods: A modified 3D ion trap was coupled to a CO2 laser and an OPO/OPA system operating in the 2300 to 4000 cm-1 range. DFT vibrational frequency calculations were carried out to support spectral assignment. Results: Detailed descriptions of the interface between the laser and the mass spectrometer, the hardware to control the laser systems, the automated system for IRMPD spectrum acquisition and data This article is protected by copyright. All rights reserved. management are presented. The optimization of the crystal position of the OPO/OPA system to maximize the spectroscopic response under low power laser radiation is also discussed. Conclusions: OPO/OPA and CO2 laser-assisted dissociation of gas ions were successfully achieved. The system was validated by acquiring the IRMPD spectra of model species and comparing with literature data. Two isomeric alkaloids of high economic importance were characterized to demonstrate the potential of this technique, which is now available as an open IRMPD spectroscopy facility in Brazil.
Infrared Spectra of Isolated Protonated Polycyclic Aromatic Hydrocarbons: Protonated Naphthalene
Angewandte Chemie International Edition, 2007
Gas-phase infrared (IR) spectra of larger protonated polycyclic aromatic hydrocarbon molecules, H + PAH, have been recorded for the first time. The ions are generated by electrospray ionization and spectroscopically assayed by IR multiple-photon dissociation (IRMPD) spectroscopy in a Fourier transform ion cyclotron resonance mass spectrometer using a free electron laser. IRMPD spectra of protonated anthracene, tetracene, pentacene, and coronene are presented and compared to calculated IR spectra. Comparison of the laboratory IR spectra to an astronomical spectrum of the unidentified IR emission (UIR) bands obtained in a highly ionized region of the interstellar medium provides for the first time compelling spectroscopic support for the recent hypothesis that H + PAHs contribute as carriers of the UIR bands.
Infrared multiple photon dissociation in the quadrupole ion trap via a multipass optical arrangement
Journal of the American Society for Mass Spectrometry, 1994
The design of a novel multipass optical arrangement for use with infrared multiple photon dissociation (IRMPD) in the quadrupole ion trap is presented. This design circumvents previous problems of limited IR laser power, small IR absorption cross sections for many molecules, and the limited ion statistics of trapping and detection of ions for IRMPD in the quadrupole ion trap. In contrast to previous designs that utilized the quadrupole ion store, the quadrupole ion trap was operated in the mass selective instability mode with concurrent resonance ejection. The instrumental design consisted of a modified ring electrode with three spherical concave mirrors mounted on the inner surface of the ring. This modified design allowed for eight laser passes across the radial plane of the ring electrode. IRMPD of protonated bis(2-methoxyethyllether (diglyme) was used to characterize the performance of the multipass ring electrode. Two consecutive reactions for the IRMPD of protonated diglyme were observed with a lower energy channel predominant at less than 0.6 J (irradiation times from 1 to 30 ms) and a second channel predominant at energies greater than 0.6 J (irradiation times > 30 ms). Other studies presented include a discussion of the dissociation kinetics of protonated diglyme, the use of a pulsed valve for increased trapping efficiency of parent ion populations, and the effects of laser wavelength and of ion residence time in the radial plane of the ring electrode on photodissociation efficiency. (1 Am Sot Mass Spectrom 1994, 5, 886-893) 0 ver the past 15 years photodissociation has become an integral tool in the study of gasphase ion chemistry [l, 21. The combination of mass spectrometry (employing both ion trap and ion cyclotron resonance instruments) and photodissociation has been used successfully to investigate the chemical kinetics, reactivity, and spectroscopy of various ionic species [3-91. The long storage times and instrumental configuration of trapping instruments are ideally suited for photodissociation experiments. Some capabilities of trapping instruments include the measurement of photon-induced ion decay as a function of laser irradiance time, the use of the multiphoton absorption process to study fragmentation, and the use of the photodissociation spectrum as a fingerprint for determination of isomeric ion structures [ 101.