Prof. Nizkorodov's Graduate and Postdoctoral Research

Caltech

JILA

Uni Basel

NSU

NO₂ cross-sections

Postdoctoral Research at Caltech

My primary Caltech research project aimed at detecting and characterizing a molecule known as HOONO. This elusive isomer of nitric acid and suspected intermediate of the OH + NO₂ + M association reaction is calculated to be bound by some 19 kcal/mol. HOONO has been observed in solid Argon matrices, and there was a compelling evidence for its transient existence in aqueous solutions. However, no one has ever seen it in the gas-phase, and not for the lack of trying! Because of its small binding energy, HOONO should readily decompose back into OH and NO₂ upon 2v₁ OH stretch excitation providing one with a very sensitive and potentially background-free way of detecting it. Indeed, Paul Wennberg and I succeeded [37] in observing the first photodissociation gas-phase spectrum of HOONO and estimated its yield in the OH + NO₂ reaction (5%). From an experimental kinetics perspective, the successful observation of HOONO by photodissociation spectroscopy under ambient temperature conditions opens the door to many exciting studies of reactions involving weakly-bound adducts such as, for example, ROONO and RO₂-H₂O (R = organic radical). Later on, Brian Bean, Andrew Mollner, Mitchio Okumura, Stanley Sander, and myself detected [39] this molecule using cavity ring-down spectroscopy and provided a more qualitative measurement of the yield as a function of temperature and pressure. Still later on, Julie Fry, John Crounse, Coleen Roehl, and Paul Wennberg measured thermal lifetime of HOONO [41].

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Postdoctoral Research at JILA

I was responsible for three research projects in Prof. Nesbitt's group at JILA (University of Colorado at Boulder and NIST). The first one was concerned with the state-resolved dynamics of chemical reactions involving highly reactive fluorine atoms studied in crossed supersonic jets using high-resolution infrared direct absorption spectroscopy. The most significant example from that work was the reaction F + H₂ -> HF(v, J) + H and its isotopic variant, F + HD. Using a combination of an ultrasensitive detection of HF, single-collision environment of the crossed jets, and a high quantum state selectivity we succeeded in measuring the full nascent HF(v,J) ro-vibrational state distribution for the first time [27]. In addition, our study provided the first definitive observation of non-adiabatic (Born-Oppenheimer forbidden) reaction between the spin-orbit excited fluorine atom and H₂, predicted by many theoreticians but never observed in an experiment [29,30]. Figure on the right shows a schematic diagram of the relevant potential energy surfaces involved in this process. As a step towards understanding the dynamics of atom-polyatom processes we also undertook a study of an atom-pentatom reaction F + CH₄ -> HF(v, J) + CH₃ at a similar level of quantum precision and detail [31,33].

This figure shows the relevant potential energy surfaces involved in the F + H₂ reaction. Non-adiabatic reaction starts on the upper (repulsive) potential energy surface.

The second project was related to the kinetics and dynamics of reactions involving hydroxyl radical (OH) aimed to improve our understanding of the intricate chain of radical-driven chemical processes occurring in the Earth's atmosphere. We undertook a detailed study of the OH/HO₂/O₃ chain reaction kinetics in a broad range of temperatures (190-320 K) using flash-photolysis and direct absorption infrared spectroscopy [32]. This reaction cycle is responsible for up to 50% of the natural ozone decomposition at mid-latitudes and accurate rate constants we obtained in that study are now used in the latest NASA compilation of kinetic parameters for atmospheric modeling.

The third project was centered around the dynamics of quantum state-resolved photodissociation of water and water-based Van der Waals complexes. In this challenging three-laser experiment, either free or bound water molecules are prepared in a specific quantum state via direct overtone pumping, and subsequently photodissociated with ultraviolet radiation. The quantum state distribution of the resulting OH fragment is thoroughly interrogated by laser induced fluorescence [38]. This study have resulted in an unprecedented amount of detailed information on vibrationally mediated dissociation of H₂O, Ar-H₂O, and water dimer. Observation of the first rotationally resolved (H₂O)₂ overtone transition represents the most exciting finding of that work [42].

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Graduate Research at the University of Basel

During my graduate career at the Institute of Physical Chemistry in Basel (advised by Evan Bieske, Otto Dopfer, and John Maier ), I was involved in studies of the structure, reactivity and dynamics of weakly-bound ionic complexes and clusters by means of ion photodissociation spectroscopy [link to papers on ion complexes]. In this method, the ions of interest are mass-selected with a quadrupole mass spectrometer and injected into an ion trap where they are photofragmented via their electronic or infrared transitions. The charged photofragments are separated from the parent ions with another mass-spectrometer and detected in essentially background-free fashion. With this form of action spectroscopy we were able to obtain unique spectroscopic, dynamical and structural information about a number of ionic complexes for the first time. Systematic studies of proton-bound complexes of the type A--HB⁺ (A = He, Ne, Ar, H₂; B = CO, N₂, H₂O, NH₃, etc.) led to an improved understanding of bonding mechanisms in these intriguing systems. We established a number of useful correlations between the proton bridge stability and the proton affinities of individual bases comprising the complex. More specifically, we found that such proton-bound complexes favor a relatively rigid linear A--H-B arrangement, with the strongest hydrogen bonds formed between bases with similar proton affinities. For example, binding energy of Ar-HN₂⁺ complex determined by our group is as large as 2781.5+/-1.5 cm^-1, which, incidentally, represents one of the most precise reported measurements of bond strength in a weakly-bound system. Larger disparity in the proton affinities leads to progressively weaker H-bridging and floppier structures up to the extreme case of He-NH₄⁺ complex, which is characterized by essentially free rotation of the NH₄⁺ core ion and a binding energy of less than 100 cm^-1. Studies of larger proton-bound complexes (A_n--HB⁺) successfully addressed the questions of cluster growth, differential solvation, and connection between microscopic and bulk properties of solvated ions. For example, we found several easily recognizable spectroscopic signatures of shell growth in Ar_n--HCO⁺, Ar_n--NH₄⁺ and similar systems.

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Undergraduate Research at the Novosibirsk State University

PI's undergraduate research in the Institute of Chemical Kinetics and Combustion in Novosibirsk, Russia, was concerned with radiationless transitions in nitrogen dioxide induced by a magnetic field [1] and with photochemistry of electronically excited NO₂ [4]. The most interesting finding of that work was the reaction of electronically excited NO₂ with the ground state NO₂, which, in addition to the standard excitation quenching route, produced significant yields of NO₃ radical [4]. The work was done in the laboratory of Prof. Nikolai Bazhin and it was supervised by Dr. Vladimir Makarov

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NO₂ high-resolution absorption cross-section data

Absolute absorption cross-sections of NO₂ measured with a high-resolution Fourier transform spectrometer at Kitt Peak National Solar Observatory

Work Info: The spectra were recorded [40] in synthetic air at a 0.060 cm^-1 resolution in the 415-525 nm range with a high-precision Fourier-transform spectrometer. The measurements were conducted for a range of pressures (1-760 Torr) and temperatures (220-298 K) that are representative of typical tropospheric and stratospheric conditions. Maximum uncertainty for the reported absolute absorption cross sections is 7% (two sigma), which is primarily limited by the light source drifts and by uncertainties in NO₂ concentrations. Note that the error is likely to be largest at the edges of the spectral range studied here; 5% is probably a better conservative estimate for the middle of the investigated range. The wavelength (referred to vacuum) accuracy is 0.011 cm^-1 (2.8x10^-4 nm at 500 nm) and precision is 0.0022 cm^-1 throughout the investigated wavelength range.

File Info: Files are saved as individual columns of ASCII text data with 4 significant digits of precision. Absorption cross section units are cm²/molecule; natural logarithm base. Vacuum wavenumber scale in cm^-1 is saved in a separate file with 10 significant digits of precision (download). Alternatively, the vacuum wavenumber scale can be reconstructed from the following end values: Start = 18000.023179375 cm^-1 ; End = 24499.973540343 cm^-1. All files are compressed using "ZIP" standard.

You can also download I₂ calibration scans 1 and 2, and a combined I₂ / NO₂ calibration scan.

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