Dr Julia H. Lehman
- Position: University Academic Fellow
- Areas of expertise: physical chemistry; vibrational and electronic spectroscopy; gas-phase reaction kinetics and dynamics; photodissociation dynamics; velocity map imaging; frequency comb spectroscopy.
- Email: J.Lehman@leeds.ac.uk
- Phone: +44(0)113 343 4260
- Location: 1.11 School of Chemistry
- Website: Twitter | ORCID
I obtained my PhD with Prof. Marsha Lester at the University of Pennsylvania in 2013, with a thesis title "Excited state dynamics of atmospheric species: Collisional quenching of OH A 2Σ+ and photodissociation of CH2I2 and CH2OO." I was then a postdoctoral researcher at JILA, University of Colorado, Boulder under the mentorship of Prof. Carl Lineberger from 2013-2016. I joined the University of Leeds in January 2017 as a University Academic Fellow.
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. 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, and potentially very sensitive (ppm or even ppb levels). A vibrational 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.
- PhD in Chemistry, University of Pennsylvania
- BS in Chemistry and Mathematics, Muhlenberg College
Research groups and institutes
- Atmospheric and Planetary Chemistry