Making molecules dance: Preparing coherent states in polyatomic radical cations


Recent technological advances, specifically the development of the femtosecond pump-probe techniques, have revolutionized our understanding of molecular dissociation processes. Application of the femtosecond pump-probe has facilitated the direct observation of coherent vibrational states, or “wave-packets” with sub-40 fs oscillation periodswith revivals observable for as long as 30 ps in the cations of diatomic molecules. However, measurements on polyatomic cations typically excite only low-frequency bending or torsional modes with oscillation periods of ~300–1000 fs. As a consequence, in many experiments only 2–6 oscillation periods in transient ion yields are visible above the noise level. Moreover, a critical challenge to measuring wave-packet dynamics in polyatomic cations arises because it is quite tedious to prepare a well-defined initial coherent state due to the prevalence of competing dissociation processes resulting from non-adiabatic multi-electron excitation at the typical laser wavelength of 800 nm. Therefore, there is need to improve the coherent state preparation in polyatomic molecules.

Recently, a team of researchers led by Dr. Katharine Moore Tibbetts from the Department of Chemistry, Virginia Commonwealth University proposed a study whose main objective was to demonstrate the preparation of a well-defined coherent state in the dimethyl methylphosphonate radical cation (DMMP+) with an oscillation period of 45 fs, which is significantly faster than any previous oscillations observed in coherent states of polyatomic radical cations. Their work is currently published in the research journal, Physical Chemistry Chemical Physics.

The research methodology employed by Virginia Commonwealth Universityscientists began with the preparation of pump and probe pulses from the output of a commercial titanium:sapphire regenerative amplifier. Next, the strong field ionizing pump pulseswere either taken directly from the 800 nm output or an optical parametric amplifier producing tunable near-IR pulses at 1200-1600 nm. The resulting wave-packet dynamics in the output radical cation were probed with a weak 800 nm pulse.The time delay between pump and probe pulses was scanned in steps of 5 fs with a motorized delay stage.

The authors observed that the oscillations in the transient ion possessed a period of 45 fs, which was seen to be at least 3 times faster than any previously observed oscillations in polyatomic radical cations. Furthermore, the use of 1200–1600 nm, as opposed to 800 nm, wavelengths for ionization increased the oscillation amplitude by a given factor and doubled the number of visible oscillation periods, thereby indicating that an adiabatic ionization mechanism considerably enhanced preparation of the coherent state. Lastly, the researchers realized that the coherent motion was assigned to a bending mode in DMMP+ with a given frequency range based on the results of the density functional theory calculations.

In conclusion, Dr. Katharine Tibbetts and colleagues have measured coherent vibrational dynamics in the DMMP+ radical cation with a significantly faster oscillation period of 45 fs, which is significantly faster than any previously reported oscillation period in the literature for polyatomic ions. They reported that at least 12 oscillation periods were visible in through the DMMP+ transient yield when the laser excitation wavelength was sufficiently long such that an adiabatic ionization mechanism operated. Altogether, their study raised the possibility that dissociation pathways in related chemical warfare agents may be actively controlled via coherent control schemes, which could in return, lead to the advancement of new technologies for chemical warfare agent detection and obliteration.

Making molecules dance: Preparing coherent states in polyatomic radical cations - Advances in Engineering

About the author

Katharine Moore Tibbetts is an Assistant Professor of Chemistry at Virginia Commonwealth University, Richmond, VA, USA. She obtained her Ph.D. in Chemistry (2012) from Princeton University (Princeton, NJ, USA) and worked as a postdoctoral researcher at Temple University (Philadelphia, PA, USA) from 2012-2015.

Her current research interests focus on two areas: (1) ultrafast dynamics of polyatomic radical cations with a focus on energetic and biologically relevant molecules, and (2) laser-assisted synthesis of metal and metal-oxide nanomaterials for catalysis applications.


Derrick Ampadu Boateng, Gennady L. Gutsev, Puru Jenac, Katharine Moore Tibbetts. Ultrafast coherent vibrational dynamics in dimethyl methylphosphonate radical cation. Phys.Chem.Chem.Phys., 2018, volume 20, page 4636.

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