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J. Phys. Chem. A 2010, 114, 1492–1497
Wavelength and Time-Resolved Imaging of Material Ejection in Infrared Matrix-Assisted Laser Desorption† Xing Fan and Kermit K. Murray* Department of Chemistry, Louisiana State UniVersity, Baton Rouge, Louisiana 70803 ReceiVed: August 10, 2009; ReVised Manuscript ReceiVed: NoVember 30, 2009
The dynamics of glycerol ablation at atmospheric pressure was studied using fast photography. A mid-infrared optical parametric oscillator was used to irradiate a droplet of glycerol at normal incidence. The wavelength of the infrared source was tunable and was varied between 2.7 and 3.5 µm for the studies. After an adjustable delay, an excimer pumped dye laser was used to illuminate the expanding plume, and the 90° scattered light was imaged with a high-speed CMOS camera. The time delay between the IR and UV lasers was varied up to 1 ms with a particular emphasis in the early stages of plume evolution below 1 µs. The threshold fluence for plume formation varied between 1000 and 6000 J/m2, and the minimum fluence corresponded to the OH stretch absorption of glycerol near 3.0 µm, which also corresponded to the greatest scattered light signal and duration of material emission. The velocity of the expanding plume was measured and ranged from >300 m/s near the OH stretch absorption to 93%, Newport, Irvine, CA) and an output coupler (70% R, Melles Griot, Albuquerque, NM). An approximate 1 mm thick and 6 mm in diameter deposit of liquid sample was prepared by dropping 3.0 µL of neat glycerol (99%, Sigma, St. Louis, MO) onto a stainless steel sample target, which was mounted on an xyz stage. The pulsed IR beam was directed through a 254 mm focal length (at 3 µm) CaF2 lens and irradiated the target at normal incidence. The IR laser was attenuated by inserting optical flats into the beam, and the pulse energy was measured using a pyroelectric detector (ED-104AX, Gentec, Palo Alto, CA). The dye laser was oriented perpendicular to the ablation laser and parallel to the sample surface to obtain the desired time resolution on the nanosecond and microsecond levels. At different time delays after the IR OPO pulse, the scattered light from the glycerol plume was recorded by the digital camera. Timing jitter was
