Photolysis of Solid Nitrogen: Threshold of Formation of N3
Sheng-Lung Chou (周勝隆), Jen-Iu Lo (羅仁佑), Meng-Yeh Lin (林孟曄), Yu-Chain Peng (彭鈺謙), Hsiao-Chi Lu (盧
曉琪), and Bing-Ming Cheng (鄭炳銘)*
National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu, Taiwan
[email protected]
Abstract
Irradiation of pure solid dinitrogen at 3 K with vacuum-ultraviolet light from a synchrotron in region 105-150 nm
produced infrared absorption lines of product l-N3 at 1657.8 and 1652.6 cm-1. The threshold wavelength to generate lN3 was determined to be 143.7±1.8 nm, corresponding to energy 8.63±0.11 eV. Quantum-chemical calculations support
the formation of l-N3 from the reaction N2 + N2, possibly through an activated complex l-N4 upon photoexcitation with
energy above 8.63 eV. Our results provide a prospective application to prepare highly energetic dense materials
involving polynitrogen molecules and an understanding of the nitrogen cycle in astronomical environments.
Keywords - Azide radical, Photochemistry, Polynitrogen molecules, Astronomical
Introduction
The photodissociation of molecular nitrogen maintains
interest because of its importance in the atmosphere of
Earth and various other astronomical environments; its
photolysis illustrates a range of problems in molecular
dynamics. The photochemistry of gaseous molecular
nitrogen has been investigated intensively, but the
corresponding knowledge in a solid phase is lacking. In
cold astronomical environments, molecular nitrogen
subsists as a solid. Photochemical reactions of nitrogen
are initiated with the breaking of the strong N-N bond.
Chemical paths in the solid state might differ from those
in the gaseous phase because according to a phenomenon
in the solid state called the cage effect. The cage effect
might not only influence the escape of a nitrogen atom
but also provide a reaction path to form a polynitrogen
species. Three known nitrogen molecules include N2, N3
and N4 [1]. Can a polynitrogen species containing
nitrogen atoms more than two be formed upon
photoexcitation in the solid state? To investigate the
photodissociation of N2 in the solid state is thus of
interest.
Experiments
For photolytic dissociation, it is important to know the
electronic state and the magnitude of the absorption of
solid N2 involved in the radiant excitation. For this
purpose, we measured the absorption spectrum of pure
solid nitrogen in the vacuum- ultraviolet (VUV) region to
the limit of transmission of optical components, about
105 nm, with a double-beam absorption apparatus[2]. N2
was deposited on a LiF window maintained at 3 K (Janis
RDK-415 cryostat). The light was dispersed from
beamline BL03 at National Synchrotron Radiation
Research Center (NSRRC, Taiwan) with a 6-m
cylindrical grating monochromator. The dispersed light
passed through the LiF substrate on which the sample
was deposited; the transmitted light impinged on a glass
window coated with sodium salicylate of which the
fluorescence was detected with a photomultiplier tube
(Hamamatsu R943) in a photon- counting mode.
After obtaining the information of pure solid nitrogen
in the vacuum- ultraviolet (VUV) absorption, we
investigate the photoexcitation of solid nitrogen in
wavelength range 110-150 nm. We selected the radiation
from an undulator with periodically spaced permanent
magnets (90 mm, U90) attached to beamline BL21A2 at
NSRRC. Harmonics from the undulator were suppressed
through absorption by Ar at pressure 1.33 kPa and a filter
window -- LiF for 105-125 nm and CaF2 for 125-150 nm.
Gaseous N2 was deposited on a CsI window cooled to 3
K (closed-cycle refrigerator, Janis RDK-415). The
experimental apparatus was similar to that described
previously [3]. We recorded the infrared absorption
spectrum of the deposited solid nitrogen samples before
and after photolysis with radiation at a selected
wavelength. The IR absorption spectrum was scanned
from 500 to 5000 cm-1 with an interferometric
spectrometer (Bomem DA8, KBr beamsplitter, HgCdTe
detector cooled to 77 K, 400 scans at resolution 0.5 cm-1,
uncertainty of wavenumber measurement ±0.05 cm-1)
Results
Fig 1 shows the VUV absorption spectrum of pure
solid N2 at 3 K in the spectral region between 106 nm and
160 nm at spectral resolution 0.1 nm; the thickness of the
deposited sample was about 8-10 μm. Inspection of the
total absorption profile and of the observed vibrational
progressions reveals that two dominant electronic
transitions include the Lyman-Birge-Hopfield (LBH, a
1
Πg←X 1Σg+) and Tanaka (TA, w 1Δu ← X 1Σg+) systems
in this region.
Fig 1. Absorption spectrum of pure solid dinitrogen in
wavelength range 105-160 nm at 3 K: resolution 0.1 nm
and step 0.1 nm were used in the measurements. The
thickness of the solid nitrogen film is estimated to be 810 μm.
In Fig 2.The absorption lines of N3 clearly appeared
after irradiation at wavelengths 122.4, 130.0, 138.7 and
142.0 nm, but not at 145.5 nm, the corresponding
infrared absorption spectra are shown as curves (a), (b),
(c), (d) and (e), respectively, even for irradiation
prolonged more than 30 min. In these experiments, the
VUV radiation was directed from the undulator U90 and
regulated by changing its gap. This operating mode yields
a beam of ultraviolet light of relative width 2 %,
corresponding to a width 0.175 eV of photon energy at
the selected wavelength. According to Fig 2, the
threshold for formation of N3 is hence between
wavelengths 142.0 and 145.5 nm, corresponding to
photon energies 8.731 and 8.521 eV, respectively. We
thus derive the threshold energy to generate N3 from
photolysis of solid nitrogen at 3 K to be 8.63±0.11 eV
(143.7±1.8 nm) as the midpoint of those two energies.
Fig 2. Infrared spectra of solid nitrogen at 3 K after
photolysis at wavelength (a) 122.4 nm; (b) 130.0 nm; (c)
138.7 nm; (d) 142.0 nm; (e) 145.5 nm for 30 min;
resolution 0.5 cm-1.
Discussion
From our results of threshold energy 8.63±0.11 eV to
form N3 is less than the dissociation energy 9.7977 eV of
gaseous N2. This result indicates that a mechanism for
formation N3 might involve reactions (1) and (2).
N2 + N2 + hν → N4 (1)
N4 + hν →N3 + N (2)
Induced by radiant excitation, two nitrogen molecules
generate N4 of which subsequent photolytic dissociation
of N4 yields N3 radical. In this possible route,
intermediate N4 plays a key role. Tetranitrogen N4 is so
important among polynitrogen molecules that it has
attracted theoretical [4] and experimental interest. To
understand the potential-energy profiles of the reaction,
we calculated the energetics of the reaction paths for
reaction N2 + N2 through activated complex tetranitrogen
N4. All calculations were performed with program
Gaussian 09; geometrical parameters, vibrational
wavenumbers and infrared intensities were initially
calculated and subsequently characterized with the
B3LYP method and both local and nonlocal terms in
conjunction with basis set aug-cc-pVTZ (correlationconsistent polarized valence triple-zeta). To enhance the
reliability of relative energies, we calculated single-point
electronic energies for the stationary points with coupledcluster theory CCSD (T). By this means, the calculations
were performed at points selected to provide a balanced
global representation of the potential-energy surface
along various reaction paths illustrated in Fig 3. TS1, l-N4,
is notable; the value is near the threshold energy
8.63±0.11 eV to form N3. We therefore propose that l-N4
might form from two nitrogen molecules upon
photoexcitation above energy 8.63 eV, with subsequent
photolytic dissociation of this transient species to produce
l-N3; this reaction path is supported by our calculations.
Fig 3. Energetics of reaction paths for the reaction N2 +
N2; in which, the energies are giving in J/mol (eV in
parentheses). The structures were optimized with
B3LYP/aug-cc-pVTZ; their energies were calculated with
basis set CCSD(T)/aug-cc-pVTZ.
The applications of our results require further
exploration. In this particular case, the consequences
might be applicable to understand the nitrogen cycle in
astrophysical science and the preparation of highly
energetic dense materials (HEDM) [5]. We have shown
that the photolytic dissociation of solid nitrogen with
energetic photons forms N3, possibly through an
intermediacy
of
polynitrogen
molecules.
The
investigation of the formation and chemistry of these
polynitrogen species enhances our understanding of the
evolution of the nitrogen and bio-nitrogen cycles in space.
Acknowledgement
National Science Council of Taiwan (contract NSC992113-M213-003-MY3) and National Synchrotron
Radiation Research Center provided financial support.
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