Article pubs.acs.org/JPCA
Repopulation of Nitrogen Excited Triplet State Following LaserInduced Filamentation Bradley R. Arnold,*,†,‡ Stephen D. Roberson,† and Paul M. Pellegrino† †
RDRL-SEE-O, U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783, United States Department of Chemistry and Biochemistry, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, United States
‡
ABSTRACT: Laser-induced filamentation was used to study the dynamics of excited molecular nitrogen decay processes. It is well-known that upper excited nitrogen triplet states can be repopulated at time delays far longer than their fluorescence lifetimes. Examination of the time-resolved emission from several different species indicates that there are two major mechanisms acting to repopulate the N2(C3Πu) excited state. The results implicate dissociative electron recombination with the nitrogen cation dimer, N+4 , and energy pooling between two N2(A3Σ+u ) triplet states as the main pathways to repopulate the emissive upper triplet state. The densities of N2(A3Σ+u ) and free electrons produced during filamentation were measured under atmospheric pressures in nitrogen and estimated to be [N2(A3Σ+u )]0 = 3 × 1015 cm−3 and [e−]0 = 3 × 1013 cm−3. The methods outlined in this report could find significant utility in measuring the concentration profiles of these important reactive intermediates within laser-induced filaments produced under different conditions.
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INTRODUCTION Nitrogen gas has been the subject of numerous spectroscopic studies since its discovery in the 18th century. More than 50 different excited states have been identified and characterized, including states of singlet, triplet, and quintet multiplicities.1−4 Experimental techniques used to study the spectroscopy of these systems include flames,3,5 electric discharges and arcs,3,6,7 high voltage electron impact,3,8,9 radio frequency absorption,3,10 and more recently laser-induced breakdown11,12 and filamentation.13 It is well established that specific excitation methods result in differing populations of excited species and therefore different types of emissions being observed.1,4 Of the most readily observed and fully characterized emissions from excited nitrogen are the first and second positive systems originating from the N2(B3Πg) and N2(C3Πu) triplet states, respectively.1−4 The mechanism for populating these excited triplet states under conditions of laser-induced filamentation has been examined previously,14−19 and it has been suggested that collision-induced intersystem crossing from a short-lived singlet state is responsible for the initial population of N2(C3Πu).14,17,18 The N2(B3Πg) triplet would then be populated either directly through collision-induced intersystem crossing or as part of the radiative cascade from N2(C3Πu). The lifetimes of N2(B3Πg) and N2(C3Πu) excited states at atmospheric pressure in nitrogen are relatively short due to collisional quenching processes: 0.4 and 1.4 ns, respectively.7,20 Along with the prompt emission from N2(C3Πu) and N2(B3Πg) excited states, delayed emission has also been reported under electron impact21 and laser-induced filamentation conditions.14 In these studies, emission from the N2(C3Πu) triplet state was observed many lifetimes after the excitation had © 2014 American Chemical Society
passed. It was concluded that, in addition to prompt decay, alternate mechanisms for the production of these excited states through the interactions of long-lived excitation carriers must also occur. Such mechanisms for repopulating these excited states would lead to delayed emission at delay times far exceeding the normal lifetime of the species involved. Several long-lived intermediates and excited states of nitrogen have been proposed that could be produced during filamentation and could serve as excitation carriers within the excited state manifold of nitrogen.21 Among the possible longlived intermediates are the lowest triplet state, N2(A3Σ+u ), and the ground state of the cation, N+2 (X2Σ+g ). There are also several long-lived excited states of differing symmetries and multiplicities that could be involved as excitation carriers as well. In addition to simple molecular excited states, the possibility of bimolecular excimer22,23 and cation dimer species24 must also be considered, particularly at the high nitrogen pressures used in this study. This report outlines our investigation of the source of excitation carriers responsible for repopulating the molecular nitrogen excited states during laser-induced filamentation. Time-resolved emission spectroscopy was used to monitor the decay of emission from the N2(C3Πu) triplet state and several other reactive intermediates. Kinetic models based on known rate constants were used to examine the observed Special Issue: Current Topics in Photochemistry Received: April 30, 2014 Revised: June 19, 2014 Published: June 19, 2014 10456
dx.doi.org/10.1021/jp504237p | J. Phys. Chem. A 2014, 118, 10456−10463
The Journal of Physical Chemistry A
Article
emission in detail and to describe the dynamics of repopulating N2(C3Πu) long after the laser pulse has passed.
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EXPERIMENTAL SECTION The laser system used has been described in detail previously.14 Briefly, a Coherent Micra femtosecond oscillator coupled with a Legend Elite USP regenerative amplifier was used to produce laser pulses centered near 800 nm with near-transform limited pulses ∼40 fs in duration and pulse energies of 2.6 mJ at 1 kHz repetition rates. The compression stage of the amplifier was adjusted to produce long, intense filaments under the specific conditions used in these experiments. The resulting laser pulses used in these studies were ∼70 fs in duration with negative group velocity dispersion. These pulses produced single stranded filaments, as confirmed by the appearance of a conical emission pattern.13 The laser pulses were directed through the center of an optical cage system (Thor Laboratories) using low GVD broadband dichroic mirrors, and the 6 mm diameter beam was focused using a 25 mm diameter quartz lens with a 0.5 m focal length. Collection optics were attached to a two inch sample cube that could slide axially along the cage structure to examine the filament at different locations along its length. For the current study, the collection optics were placed 0.5 m from the quartz focusing lens, near the geometric focus of the laser. Gas samples were introduced through the top of the cube. Nitrogen gas from in-house liquid nitrogen boil-off with ultrapurification (N2 >99.9999%, O2
