Physics Colloquium
 
Dr. Robert J.  Levis
Center for Advanced Photonics Research
Department of Chemistry
Temple University


   
Is there Chemistry at 10¹³ W cm^-2?
Condensed Phase Laser Vaporization for Universal Mass Analysis, Laser Filamentation
for Standoff Detection, and Strong Field Electronic State Spectroscopy

102 Cardwell Hall
May 6, 2013
4:30 p.m.

The interaction of complex molecules with an ultra fast, ultra intense (10¹³ W cm^-2) near IR (800 nm) laser enables new experiments for physics and chemistry. In this regime, the laser pulse has electric fields (V/A) approaching those binding electrons to nuclei and the short pulse duration (50 fs) prevents the intuitively-expected catastrophic destruction of polyatomic molecules. In this talk, we will first consider the interaction of ultra intense, ultrafast radiation condensed phase systems with van der Waals binding. The interaction of intense lasers with such systems results in a universal vaporization method that provides a new, atmospheric pressure release method for samples with molecular weight exceeding 10⁵ amu. A universal analysis method results when femtosecond laser vaporization is combined with electrospray ionization. The method, laser electrospray mass spectrometry or LEMS, has been used to classify improvised explosive device (IED) formulations, pharmaceuticals, lipids, plant phenotype and even condensed phase protein structure.

In second example of strong field molecular science, we will consider the propagation of an intense, femtosecond laser pulse through the atmosphere. When a 2 mJ, 50 fs pulse interacts with air, self focusing occurs and the intensity again exceeds 10¹³ W cm^-2) and laser filamentation occurs. This phenomena results in an underdense plasma string approximately 1 meter in length wherein the laser pulse self shortens to just a few optical cycles, ~5fs. The resulting white light string can be used to induce impulsive vibrational and rotational motion in molecules, which in turn enable gas phase Raman spectroscopy. Thus strong field physics has enabled a new standoff detection method for technologically interesting molecules, including signatures for IEDs.

I will conclude the talk with a new experiment that enables spectroscopic measurement of the excited electronic states of radical cations. In this experiment infrared tunnel ionization occurs at 10¹³ W cm^-2) followed by excitation from 1100nm to 1600nm to map the electronic structure of alkylphenones. This spectroscopy sheds light onto the mechanisms of strong field reaction control using shaped laser pulses, an area that has proven elusive for a decade.

Refreshments in CW 119 at 4:15 p.m.