Juraj Jašík

Probing isomers of the benzene dication in a low-temperature trap.

The structure of doubly ionized benzene has been spectroscopically studied for the first time. Helium-tagged complexes were prepared at temperatures below 4 K and analyzed using infrared predissociation spectroscopy. Double ionization of benzene yields primarily high-energy dications with a six-membered-ring structure. Some of the dications undergo rearrangement to a more stable pyramidal isomer with a C5H5 base and CH at the apex. By means of isomer-selective heating by a CO2 laser, infrared predissociation spectra of both the classical and pyramidal dications were obtained.

Metal-assisted Lossen rearrangement.

A new reaction mechanism for the Lossen rearrangement of hydroxamic acids catalyzed by basic salts is presented. It is shown that the rearrangement proceeds in metal complexes of deprotonated hydroxamic acids. The deprotonation can occur either at the oxygen atom (observed for the zinc complexes) or at the nitrogen atom (observed for the potassium complexes). Both anionic forms are characterized by infrared multiphoton dissociation spectroscopy. The rearrangements proceed from the reactive N-deprotonated metal hydroxamates and lead to metal carbamates. The mechanism is elucidated by computational chemistry, mass-spectrometric studies, and preparative experiments.

Laser-induced breakdown spectroscopy of iron oxide powder

Iron oxide Fe2O3 nano-particle powder is investigated by laser-induced breakdown spectroscopy (LIBS). Laser ablation of loose Fe2O3 powder (density 0.43 g cm−3) leads to the ejection of particles and generates a plasma channel along the laser beam path. LIBS spectra are obtained by spatially integrated and time-gated measurement in flushed N2 gas background. A simple powder delivery system allows for investigation of the nano-particle powder directly without any pre-treatment and fixation of the loose material. The Saha–Boltzmann analysis of spectra yields plasma temperature T = (10250 ± 200) K and electron density Ne = (11 ± 3) × 1016 cm−3 for the loose Fe2O3 powder and for pressed Fe2O3 powder pellets that are investigated for comparison. LIBS analysis of more than 20 different powder samples establishes calibration curves for Al, Si, Ni and Mn element impurities. The limits of detection derived from such curves are 8, 15, 66 and 860 ppm for Al, Si, Ni and Mn, respectively.

Two-Color Infrared Predissociation Spectroscopy of C6H62+ Isomers Using Helium Tagging

Two-color IR-IR isomer selective predissociation spectra of helium-tagged C6H6(2+) are presented. The dications are generated via electron bombardment of either benzene or 1,3-cyclohexadiene. After mass selection they are injected into a 2.6 K cold ion trap where the presence of a dense He buffer gas not only cools them but also leads to He attachment. The ion ensemble is exposed to one or two intense IR pulses from optical parametric oscillators (OPOs) (1200-3100 cm(-1)) before it is extracted, mass analyzed, and detected. On the basis of a comparison with theoretical predictions, the resulting spectral features allow us to separate and assign different isomers of C6H6(2+) dications. Compression of the ion cloud very close to the axis of the linear quadrupole trap and coaxial superposition of well-collimated laser beams results in the fragmentation of almost all helium complexes at specific wavelengths. This unique feature enables us to record fluence-dependent attenuation curves for individual absorption bands and thus determine not only absorption cross sections but also the composition of the ion mixture.

Spectroscopic Characterization and Reactivity of Triplet and Quintet Iron(IV) Oxo Complexes in the Gas Phase

Abstract Closely structurally related triplet and quintet iron(IV) oxo complexes with a tetradentate aminopyridine ligand were generated in the gas phase, spectroscopically characterized, and their reactivities in hydrogen‐transfer and oxygen‐transfer reactions were compared. The spin states were unambiguously assigned based on helium tagging infrared photodissociation (IRPD) spectra of the mass‐selected iron complexes. It is shown that the stretching vibrations of the nitrate counterion can be used as a spectral marker of the central iron spin state.

Chasing the Evasive Fe═O Stretch and the Spin State of the Iron(IV)–Oxo Complexes by Photodissociation Spectroscopy

We demonstrate the application of infrared photodissocation spectroscopy for determination of the Fe═O stretching frequencies of high-valent iron(IV)-oxo complexes [(L)Fe(O)(X)]2+/+ (L = TMC, N4Py, PyTACN, and X = CH3CN, CF3SO3, ClO4, CF3COO, NO3, N3). We show that the values determined by resonance Raman spectroscopy in acetonitrile solutions are on average 9 cm-1 red-shifted with respect to unbiased gas-phase values. Furthermore, we show the assignment of the spin state of the complexes based on the vibrational modes of a coordinated anion and compare reactivities of various iron(IV)-oxo complexes generated as dications or monocations (bearing an anionic ligand). The coordinated anions can drastically affect the reactivity of the complex and should be taken into account when comparing reactivities of complexes bearing different ligands. Comparison of reactivities of [(PyTACN)Fe(O)(X)]+ generated in different spin states and bearing different anionic ligands X revealed that the nature of anion influences the reactivity more than the spin state. The triflate and perchlorate ligands tend to stabilize the quintet state of [(PyTACN)Fe(O)(X)]+, whereas trifluoroacetate and nitrate stabilize the triplet state of the complex.

Infrared and Visible Photodissociation Spectra of Rhodamine Ions at 3 K in the Gas Phase.

Helium-tagging predissociation spectroscopy in the visible spectral range (He@VisPD) is shown for Rhodamine 123, Rhodamine 110, and Rhodamine 110s silver salt (silver carboxylate). It is shown that the spectra reflect single-photon absorption. The helium-tagged ions are in the ground vibrational state, and the He@VisPD spectra feature the Franck-Condon envelopes for the excitation to the first excited singlet state that agree very well with theoretical simulations. The S0 → S1 excitation energies are 2.712 ± 0.006 eV for Rhodamine 123, 2.700 ± 0.006 eV for Rhodamine 110, and 2.751 ± 0.006 eV for the silver salt of Rhodamine 110. The determined energies can be slightly blue-shifted due to the binding energy of helium. The Rhodamine ions were also characterized by helium-tagging infrared photodissociation spectroscopy. The distinctive spectral features of the individual derivatives are described and the spectra are compared to the classical solid-state IR spectra.

Cold and ultracold reactive collisions between FeO+ and H2

The nominal temperature range of cryogenic radio-frequency ion traps has recently been extended down to T=2.3 K. Whereas in situ He tagging of mass-selected ions embedded in dense helium buffer gas is becoming common for recording IR spectra through photofragmentation of small and large ions, much less activity is devoted to the field of cold chemistry, which in this contribution means the two orders of magnitude extending from 300 to below 3 K. The importance of this temperature range for understanding the dynamics of bi- and termolecular reactions is illustrated with new results for the time-honored reaction of FeO+ with H2 obtained with the cryogenic ion trap ISORI in Prague. The rate coefficient for forming Fe+ +H2 O increases steeply with decreasing temperature. In addition more product channels open up, such as the stabilized reaction-intermediate complexes H2 FeO+ and Hen -FeO+ formed by ternary association with He. For the FeOH+ +H channel only a minor signal is observed. The rate coefficients provide deep insight into lifetimes, bottlenecks, and barriers impeding almost completely the exothermic, but spin-forbidden, reaction at room temperature. For some of the He-tagged ions, IR predissociation spectra are recorded. A breakthrough is obtaining the first spectrum of [(H2 )FeO]+ , synthesized and tagged in situ with He. These results pave the way to study the structures of reaction intermediates stabilized in the gas phase by means of collisions with helium.

Laser-induced optical breakdown spectroscopy of polymer materials based on evaluation of molecular emission bands

Laser-induced breakdown spectroscopy (LIBS) for composition analysis of polymer materials results in optical spectra containing atomic and ionic emission lines as well as molecular emission bands. In the present work, the molecular bands are analyzed to obtain spectroscopic information about the plasma state in an effort to quantify the content of different elements in the polymers. Polyethylene (PE) and a rubber material from tire production are investigated employing 157nmF2 laser and 532nm Nd:YAG laser ablation in nitrogen and argon gas background or in air. The optical detection reaches from ultraviolet (UV) over the visible (VIS) to the near infrared (NIR) spectral range. In the UV/VIS range, intense molecular emissions, C2 Swan and CN violet bands, are measured with an Echelle spectrometer equipped with an intensified CCD camera. The measured molecular emission spectra can be fitted by vibrational-rotational transitions by open access programs and data sets with good agreement between measured and fitted spectra. The fits allow determining vibrational-rotational temperatures. A comparison to electronic temperatures Te derived earlier from atomic carbon vacuum-UV (VUV) emission lines show differences, which can be related to different locations of the atomic and molecular species in the expanding plasma plume. In the NIR spectral region, we also observe the CN red bands with a conventional CDD Czerny Turner spectrometer. The emission of the three strong atomic sulfur lines between 920 and 925nm is overlapped by these bands. Fitting of the CN red bands allows a separation of both spectral contributions. This makes a quantitative evaluation of sulfur contents in the start material in the order of 1wt% feasible.

Spin-State-Controlled Photodissociation of Iron(III) Azide to an Iron(V) Nitride Complex

The generation of iron(V) nitride complexes, which are targets of biomimetic chemistry, is reported. Temperature-dependent ion spectroscopy shows that this reaction is governed by the spin-state population of their iron(III) azide precursors and can be tuned by temperature. The complex [(MePy2 TACN)Fe(N3 )]2+ (MePy2 TACN=N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane) exists as a mixture of sextet and doublet spin states at 300 K, whereas only the doublet state is populated at 3 K. Photofragmentation of the sextet state complex leads to the reduction of the iron center. The doublet state complex photodissociates to the desired iron(V) nitride complex. To generalize these findings, we show results for complexes with cyclam-based ligands.