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Julia H Lehman

University Academic Fellow
Member of the Computational Chemistry and Chemical Physics group

Contact details

Room: 1.55
Tel: +44 (0)113 3434260
Email: J.Lehman@leeds.ac.uk

Keywords

Physical chemistry
Vibrational and electronic spectroscopy
Gas-phase reaction kinetics and dynamics
Photodissociation dynamics
Velocity Map Imaging
Frequency Comb Spectroscopy

Photograph of Julia H Lehman

Research interests

My research group is interested in understanding the making and breaking of chemical bonds through studying the kinetics and dynamics of small molecule reactions relevant to the chemistry of planetary atmospheres. We are particularly interested in studying fundamental reactions important in atmospheric oxidation processes, such as intermediates in the OH oxidation of reduced sulphur compounds in high NOx environments forming alkyl substituted thionitrites. One of our goals is to understand how these reactions progress, how fast (or slow) they are, and what products are formed. Another goal is to understand what happens to these molecules after they form, particularly in the presence of light.

The research in my group uses a two-pronged approach with complementary instruments to study chemical transformations: (1) we obtain kinetic and spectroscopic information using frequency comb spectroscopy, a simultaneously broadband and high resolution technique; and (2) we obtain information on the electronic spectroscopy leading to the photodissociation of molecules and/or radicals using velocity map imaging and time-of-flight detection techniques.

Frequency comb lasers are an exciting new tool for chemists. A frequency comb laser is essentially a mode locked femtosecond laser where the laser cavity roundtrip time is locked to a submultiple of the laser repetition rate, generating a series of evenly spaced "comb teeth" which have a known and controllable optical frequency. Our group is specifically interested in frequency comb lasers operating in the mid-infrared. This is a particularly advantageous spectral window for chemists, which is used to identify molecules based on their vibrational absorption spectrum. Using cavity-enhanced direct frequency comb spectroscopy, a vibrational absorption spectrum can be obtained which is simultaneously broadband (covering over 400 nm in the 3 – 3.5 µm range), high resolution (comb teeth spacing of less than 0.01 cm-1), and is potentially very sensitive (ppm or even ppb levels). The absorption spectrum will be used to identify the reactant and product molecules in a reaction cell. In addition, by monitoring the broadband, high resolution absorption spectrum as a function of time after the reaction is initiated, the kinetics of the reaction can be determined. This can be taken one step further by coupling this detection method with a reaction taking place under known and controllable temperatures, yielding temperature dependent kinetics relevant to planetary chemistry or even astrochemistry.

The transformation of molecules can also take place following irradiation with light, such as the breaking of chemical bonds following photoexcitation to excited electronic states (called photodissociation). In a simple photodissociation process (such as AB + hν → A + B), two fragments are formed with some amount of internal and translational energy. Because of the conservation of energy, the amount of available energy to translational or internal degrees of freedom of the two fragments is directly related to the amount of energy put into the system by the photon minus the amount of energy it took to break the chemical bond. Velocity map imaging (VMI) is a "pump-probe" experimental technique which is able to probe the velocity distribution and angular anisotropy of a fragment, thereby giving information about the photodissociation dynamics. Often, VMI is used in conjunction with state-selective ionization techniques, such as resonance-enhanced multiphoton ionization (REMPI). This is so that the internal energy of one fragment is selected experimentally, which results in additional information being known about the internal energy of the cofragment.

Publications

Lehman JH, Lineberger WC Photoelectron spectroscopy of the thiazate (NSO-) and thionitrite (SNO-) isomer anions Journal of Chemical Physics 147 -, 2017
DOI:10.1063/1.4984129
View abstract

Nelson DJ, Gichuhi WK, Miller EM, Lehman JH, Lineberger WC Anion photoelectron spectroscopy of deprotonated ortho-, meta-, and para-methylphenol Journal of Chemical Physics 146 -, 2017
DOI:10.1063/1.4975330
View abstract

Oliveira AM, Lehman JH, McCoy AB, Lineberger WC Photoelectron spectroscopy of the hydroxymethoxide anion, H 2 C(OH)O− The Journal of Chemical Physics 145 -, 2016
DOI:10.1063/1.4963225
View abstract

Oliveira AM, Lehman JH, McCoy AB, Lineberger WC Photoelectron Spectroscopy of cis- Nitrous Acid Anion ( cis- HONO– ) The Journal of Physical Chemistry A 120 1652-1660, 2016
DOI:10.1021/acs.jpca.5b11797

Oliveira AM, Lu Y-J, Lehman JH, Changala PB, Baraban JH, Stanton JF, Lineberger WC Photoelectron Spectroscopy of the Methide Anion: Electron Affinities of• CH 3 and • CD 3 and Inversion Splittings of CH 3 – and CD 3 – Journal of the American Chemical Society 137 12939-12945, 2015
DOI:10.1021/jacs.5b07013

Lu Y-J, Lehman JH, Lineberger WC A versatile, pulsed anion source utilizing plasma-entrainment: Characterization and applications The Journal of Chemical Physics 142 044201-044201, 2015
DOI:10.1063/1.4906300

Lehman JH, Lineberger WC Visible spectrum photofragmentation of O 3− (H 2 O) n , n ≤ 16 The Journal of Chemical Physics 141 154312-154312, 2014
DOI:10.1063/1.4898373

Opoku-Agyeman B, Case AS, Lehman JH, Lineberger WC, McCoy AB Nonadiabatic photofragmentation dynamics of BrCN− The Journal of Chemical Physics 141 084305-084305, 2014
DOI:10.1063/1.4892981

Lehman JH, Lester MI Dynamical Outcomes of Quenching: Reflections on a Conical Intersection Annual Review of Physical Chemistry 65 537-555, 2014
DOI:10.1146/annurev-physchem-040513-103628

Lehman JH, Lester MI, Kłos J, Alexander MH, Dagdigian PJ, Herráez-Aguilar D, Aoiz FJ, Brouard M, Chadwick H, Perkins T, Seamons SA Electronic Quenching of OH A 2Σ + Induced by Collisions with Kr Atoms The Journal of Physical Chemistry A 117 13481-13490, 2013
DOI:10.1021/jp407035p

Lehman JH, Li H, Lester MI Ion imaging studies of the photodissociation dynamics of CH2I2 at 248nm Chemical Physics Letters 590 16-21, 2013
DOI:10.1016/j.cplett.2013.10.029

Lehman JH, Li H, Beames JM, Lester MI Communication: Ultraviolet photodissociation dynamics of the simplest Criegee intermediate CH 2 OO The Journal of Chemical Physics 139 141103-141103, 2013
DOI:10.1063/1.4824655

Lehman JH, Lester MI, Yarkony DR Reactive quenching of OH A 2Σ + by O 2 and CO: Experimental and nonadiabatic theoretical studies of H- and O-atom product channels The Journal of Chemical Physics 137 094312-094312, 2012
DOI:10.1063/1.4748376

Lehman JH, Bertrand JL, Stephenson TA, Lester MI Reactive quenching of OD A 2Σ + by H 2 : Translational energy distributions for H- and D-atom product channels The Journal of Chemical Physics 135 144303-144303, 2011
DOI:10.1063/1.3644763

Ingersoll CM, Niesenbaum RA, Weigle CE, Lehman JH Total phenolics and individual phenolic acids vary with light environment in Lindera benzoin Botany 88 1007-1010, 2010
DOI:10.1139/B10-072

Lehman JH, Dempsey LP, Lester MI, Fu B, Kamarchik E, Bowman JM Collisional quenching of OD AΣ2+ by H2: Experimental and theoretical studies of the state-resolved OD XΠ2 product distribution and branching fraction The Journal of Chemical Physics 133 164307-164307, 2010
DOI:10.1063/1.3487734