Photo-fragmentation cross-section of gaseous 2,4,6-trinitrotoluene at different ultraviolet wavelengths

Article abstract of DOI:10.1016/j.saa.2008.10.023

The photo-fragmentation cross-section of 2,4,6-trinitrotoluene (TNT) vapor at room temperature was determined at different ultraviolet wavelengths (254, 300, 340, and 400 nm) by measuring the concentration of NO molecule with cavity ring down spectroscopy

Full text of DOI:10.1016/j.saa.2008.10.023

Spectrochimica Acta Part A 72 (2009) 470–473
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Photo-fragmentation cross-section of gaseous 2,4,6-trinitrotoluene
at different ultraviolet wavelengths
b,∗
a
a
a
Ramesh C. Sharma , Tracy S. Miller , Alexander D. Usachev , Jagdish P. Singh ,
a
a
Fang-Yu Yueh , David L. Monts
Institute for Clean Energy Technology, Mississippi State University, Starkville, MS 39759-7704, United States
Department of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, United Kingdom
a
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 June 2008
Received in revised form 15 October 2008
Accepted 17 October 2008
The photo-fragmentation cross-section of 2,4,6-trinitrotoluene (TNT) vapor at room temperature was
determined at different ultraviolet wavelengths (254, 300, 340, and 400 nm) by measuring the concen-
tration of NO molecule with cavity ring down spectroscopy and correcting for the photo-fragmentation
cross-section of NO2. Nitric oxide (NO) molecules are produced by the TNT photo-fragmentation processes
via an intermediate production of NO2. Our results reveal that the photo-fragmentation cross-section of
TNT changes appreciably with change in wavelength with xenon arc lamp illumination, increasing with
decreasing excitation wavelength. The maximum value of cross-section was observed at the shortest
photo-fragmentation wavelength studied (254 nm), which is closest to the wavelength of an absorption
peak of TNT near 220 nm.
Keywords:
Cavity ring down laser spectroscopy
TNT vapor
Photo-fragmentation cross-section
Spectral dependence
©
2008 Elsevier B.V. All rights reserved.
1
. Introduction
[1–7]. Spectroscopic methods can provide high selectivity and sen-
sitivity for explosive detection [7]. An alternative approach to direct
spectroscopic detection is photo-fragmentation (PF) spectroscopy.
In this technique, characteristic fragments generated from the pho-
tolysis of the parent molecule are monitored spectroscopic ally
[8–11]. To develop a quantitative description of the PF process, it
is necessary to know the absorption cross-section of the corre-
sponding explosive materials in the gas phase. Because of their low
vapor pressures and pronounced tendency to decompose at ele-
vated temperatures, the available ultraviolet (UV) spectra of these
compounds have primarily been recorded either in solution [12–14]
in the solid phase; [14,15] gas phase spectra are [16–20]. Cavity
ring down spectroscopy (CRDS) is the most suitable technique for
study of the optical properties of these specific nitro compounds
[18]. By means of CRDS, we can measure the vapor density of TNT
in real time, while conventional methods require many hours for
vapor mass collection [19]. As a part of our efforts [18] to study
the optical properties of gaseous TNT in the ultraviolet region, the
photo-fragmentation cross-section of TNT vapor at room tempera-
ture (300 K) was evaluated at different wavelengths by using CRDS
to measure the concentration of nitric oxide (NO) molecules pro-
duced.
The optical characteristics of toxic energetic materials (EMs) is
of scientific interest because of possible health hazards as these
materials can easily penetrate the soil and then migrate into ground
water. 2,4,6-Trinitrotoluene, commonly known as TNT, is a yellow,
odorless solid that does not occur naturally in the environment.
It enters the environment in waste waters and solid waste result-
ing from the manufacture of the compound, the processing and
destruction of bombs and grenades, and recycling of explosives.
Exposure to TNT occurs through eating, drinking, touching, or inhal-
ing contaminated soil, water, or air containing TNT. The health
effects in people exposed to TNT include anemia, abnormal liver
function, skin irritation, and cataracts. This substance has been
found in at least 20 of the 1430 (1.4%) National Priorities list sites
identified by the U.S. Environmental Protection Agency (EPA). The
EPA has determined that this substance is a possible human car-
cinogen. In recent years, worldwide attention has been drawn to
efforts to detect it by several analytical methods. To decrease the
detection limit, there is a need for improved analytical performance
in terms of sensitivity, selectivity, and temporal response that are
beyond the scope of conventional methods of analysis. TNT vapor
and explosive material have been studied by several techniques
2. Experimental
∗ Corresponding author. Tel.: +44 161 306 3963.
E-mail address: Ramesh.Sharma@manchester.ac.uk (R.C. Sharma).
Fig. 1 illustrates the experimental arrangement. A dye laser
(Continuum ND60) was pumped by the third harmonic (355 nm)
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.10.023

R.C. Sharma et al. / Spectrochimica Acta Part A 72 (2009) 470–473
471
Fig. 1. Experimental arrangement for TNT photo-fragmentation and UV-CRDS to measure the resulting NO concentration.
◦
of a pulsed Nd:YAG laser (Continuum NY82S-10) with a 10-Hz rep-
etition rate. The pulse duration was about 7–8 ns. The dye laser
was operated with Coumarin 460 dye and generated 5-mJ pulses
with 0.04 cm spectral width in the 445–485 nm spectral range. To
achieve proper spatial beam quality, a 1×-Galilean telescope with
two 200-mm focal length lenses and a 100-␮m pinhole were used
together with two iris apertures.
sured using thermocouples that have an accuracy of ± 0.1 C. The
radiation of the xenon arc lamp, which entered from a direction
perpendicular to that of the laser radiation, was used to photo-
fragment TNT. The photo-fragmentation cross-section of TNT by the
xenon arc lamp was evaluated by monitoring the concentration of
NO using CRDS.
−1
To perform CRDS, the ring down resonator was built from two
mirrors, M1 and M2 (MLD Technologies), each with a 6-m radius
of curvature. The maximum reflection of the mirrors was 99.7% at
230 nm; the minimum was 99.1% as the wavelength varied ± 2.5 nm.
A photomultiplier tube (Hamamatsu R166UH) was used to record
the CRDS decay waveform. Special attention was paid to the acqui-
sition mode of the photomultiplier tube (PMT) to avoid signal
saturation. To make the receiver system less sensitive to the ring
down resonator alignment, a converging lens (labeled “collection
lens” in Fig. 1) and a quartz diffuser plate were placed before the
PMT. Signals were digitized by a digital oscilloscope (Tektronix TDS
410A) and transmitted to a data acquisition computer through a
GPIB interface. Finally, photo-fragmentation of TNT was evaluated
by using appropriate selected filter wavelengths (centered at 254,
300, 340, 400 nm with bandwidths of ∼20 nm).
The dye laser was frequency doubled by an INRAD AT-II Auto
tracker II and the output was frequency selected using an INRAD
752-104 four-prism filter (labeled “wavelength separator” in Fig. 1).
Mode matching was achieved with the help of a 5×-Galilean tele-
scope that consisted of two quartz lenses of 20- and 100-mm focal
lengths and a 10-␮m pinhole. The 227-nm laser beam entering the
−
1
ring down cavity had 0.3 mJ energy, 0.3 cm
spectral width, 1-
−1
mm diameter, and 0.75 m beam convergence. We modified the
resonance cavity used in previous work [18] and it is shown in
Fig. 1.
The laser radiation and radiation from a 100-W xenon arc lamp
(
Oriel, Model 66187) entered the cavity in mutually perpendicu-
lar directions. The cell, with a length of 52.6 cm and a radius of
.75 cm, was constructed from stainless steel high vacuum compo-
1
nents (MDC Vacuum Products). The TNT sample used was added to
the cell by dissolving 0.3 g of it in 2 mL of acetone. It was spread on
the walls of the PMT end of the cell. The walls were coated with the
solution and then the solvent was evaporated. The cell was then
evacuated and filled with argon to a pressure of one atmosphere.
3. Results and discussion
To decrease the detection limit of the energetic material using
the PF technique, the following two criteria must be fulfilled. (1) The
molecular species to be detected must have an appreciable PF cross-
section at a wavelength ꢀ that is accessible with existing lasers/light
sources. (2) Absorption of this parent molecule at ꢀ must result in
bond dissociation.
◦
Instead of higher temperatures (92 C) [18] the present experiment
was performed at room temperature. To avoid mirror condensation,
◦
the viewports were always kept 10 C above room temperature by
heating them with a heating tape. The cell temperature was mea-
Table 1
Experimental parameters.
Filter (nm)
Power (mW)
Exposure time (min)
Et (J)
[NO]con = ˛/ꢁ (per cm3)
[TNT]con (per cm3)
[N]hꢂ
2
3
3
54
00
40
0.1
154
145
168
332
0.924
1.0005
1.21
7.5E10
7.25E10
6.2E10
7.15E10
6.2E11
6.2E11
6.2E11
6.2E11
1.18E18
1.51E18
2.06E18
1.05E19
0.115
0.12
0.25
4
00
14.94

472
R.C. Sharma et al. / Spectrochimica Acta Part A 72 (2009) 470–473
Table 2
Photo-fragmentation and absorption cross-sections as a function of wavelength.
ꢁTNT-NO at RTa (cm /molecule)
2
ꢁNO –NO at 21 C (cm /molecule)
◦
b
2
ꢁPF–TNT at RTa (cm /molecule)
2
ꢁ
Ab–TNT
at 92 Cc (cm /molecule)
◦
2
Wavelength (nm)
2
2
3
3
54
00
40
9.85E−19
7.44E−19
4.75E−19
1.05E−19
1.05E−20
1.32E−19
2.02E−19
–
9.75E−18
6.13E−19
2.73E−19
9.50E−20
4.E−17
1.E−17
4
00
a
b
c
This work.
NO2 + hꢂ → NO + O: data from Ref. [21].
◦
TNT absorption cross-section at 92 C reported in Ref. [18].
Keeping these facts in mind, the present work was devoted to
the xenon lamp using the equation:
estimating the maximum PF cross-section of TNT at room temper-
ature by determining its PF cross-section at different wavelengths.
We used a 100-W xenon arc lamp along with different filters to
obtain appropriate wavelengths for photo-fragmentation of TNT
into NO. The photochemical processes involved in the present PF
of TNT are:
Et = P × t
(9)
where P is the power of the photolysis wavelengths from the xenon
arc lamp, which are selected by optical filters, and t is the exposure
time of these photolysis wavelengths to the cell. Finally, Nhꢂ was
obtained by using:
C H CH (NO ) + hꢂ
→
(Power)(time)
6
2
3
2
3
(xenonarclamp)
2
Nhꢂ
=
(10)
hc/ꢀ
C H CH (NO ) + NO (X A )
(1)
(2)
6
2
3
2
2
2
1
where h is Planck’s constant, c is the speed of light in vac-
uum, and ꢀ is the photolysis wavelength from the Xe lamp as
selected using the optical filters. The values of Et and Nhꢂ are
calculated using Eqs. (9) and (10) and are tabulated in Table 1.
Table 1 lists the different experimental parameters used for
these measurements. Table 2 lists the total photo-fragmentation
cross-sections ꢁTNT–NO for photo-fragmentation of TNT result-
2
2
1
3
NO (X A ) + hꢂ
→ NO(X ꢃ) + O( D)/O( P)
2
1
(xenonarclamp)
The presence of the resulting NO concentration is detected by
sensitive ring down laser spectroscopy by recording its absorption
signal:
2
2
+
NO(X ꢃ) + hꢂ
→ NO(A ꢄ )
(3)
(
dyelaser)
ing in NO, the photo-fragmentation cross-sections ꢁTNT
NO–NO2
for photo-fragmentation of NO2 resulting in NO [21], and the
photo-fragmentation cross-sections ꢁPF–TNT of TNT at different
excitation wavelengths of the xenon arc lamp. The different photo-
fragmentation cross-sections are related by
The total photo-fragmentation cross-section of TNT to NO,
ꢁTNT–NO, was then calculated by using the following formula:
[
NO]con × Volume
, cm2
ꢁ
=
(4)
TNT–NO
[
TNT]con × LTNT × [N]
hꢂ
ꢁ
PF–TNT(ꢀ) = ꢁTNT–NO(ꢀ) − ꢁ
(ꢀ)
(11)
NO –NO
2
where [NO]con is the concentration of NO produced by PF of TNT by
the xenon arc lamp, Volume is the total volume of the cell, [TNT]con
is the concentration of TNT in the cell at temperature T (in the
present experiment at room temperature), LTNT is the total length
of the cell, and [N]hꢂ is the number of photons having a frequency ꢂ.
The various photo-fragmentation cross-sections are plotted
as a function of the excitation wavelength in Fig. 2. It is clear
from the graph that the TNT photo-fragmentation cross-section
ꢁ
PF–TNT increases with decrease in PF wavelength. Our result is
in accordance with the result reported by Simeonsson et al. [22]
that the PF efficiency of energetic molecules is usually higher
[
TNT]con is calculated by first calculating its partial pressure using
the relationship given by Dionne et al. [19].
−
5481
T (K)
log10 P (ppb) =
+ 19.37
(5)
(6)
The [TNT]con in Eq. (4) was then estimated by using
p = [TNT]conkT
where k is Boltzmann constant and T is the temperature (in
Kelvin). The concentration of nitric oxide [NO]con, was evaluated
as described in our previous experiment [18]. We first determined
the absorption coefficient of NO molecule by measuring the CRDS
decay time (ꢅ) and using the equation
ꢀ
ꢁ
L
1
1
k(ꢀ) =
−
(7)
cl ꢅ(ꢀ)
ꢅ0(ꢀ)
where L is the resonator cavity length, l is the absorption cell length;
ꢅ0 and ꢅ are the CRDS decay times of the waveforms measured for
an empty cell and a filled cell, respectively. The concentration of
nitric oxide was evaluated using
k(ꢀ)
ꢁ
[
NO]con
=
(8)
where ꢁ is the absorption cross-section of NO molecule determined
earlier [18]. To calculate Nhꢂ, we first estimated the energy (Et) of
Fig. 2. Change in photo-fragmentation cross-section of TNT, NO2, and total TNT to
NO with wavelength. RT = room temperature.

R.C. Sharma et al. / Spectrochimica Acta Part A 72 (2009) 470–473
473
at shorter wavelengths. The increasing ꢁPF–TNT with decreasing
Acknowledgements
photo-fragmentation wavelength may be also explained (1) by
the fact that our previous study [18] has demonstrated that the
absorption cross-section of TNT vapor increases with decreas-
ing wavelength and (2) by the fact that the NO2 functionality of
TNT is easily removed by excitation in the 190–250 nm region
of the ultraviolet spectrum [15] and that this NO2 molecule
is further photo-fragmented to form NO. As Fig. 2 shows the
photo-fragmentation cross-section of TNT is much larger than the
This work was supported by U.S. Army Research Office contracts
ARO DAA HO4-961-0137 and DE-FG02-93 CH-10575.
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