Several celestial objects, including UV rich regions of planetary and reflection nebulae, stars, H II regions, and extragalactic sources, are characterized by the unidentified infrared emission bands (UIR bands). A few years ago, it was proposed that polycyclic aromatic hydrocarbon species (PAHs) are responsible for most of the UIR bands. This hypothesis is based on a spectrum analysis of the observed features. Comparisons of observed IR spectra with lab absorption spectra of PAHs support the PAH hypothesis. An example spectrum is represented, where the Orion Bar 3.3 micron spectrum is compared with the absorption frequencies of the PAHs Chrysene, Pyrene, and Coronene. The laser excited 3.3 micron emission spectrum is presented from a gas phase PAH (azulen). The infrared fluorescence theory (IRF) is briefly explained, followed by a description of the experimental apparatus, a report of the results, and discussion

Barker, John R.

Cherchneff, Isabelle

English

NASA Technical Reports Server

N9 1 - 14 93_
INFRARED FLUORESCENCE FROM PAHs IN THE LABORATORY
Isabelle Cherchneff and John R. Barker
Department of Atmospheric, Oceanic and Space sciences
Space Physics Research Laboratory
The University of Michigan, Ann Arbor, M148109-214 3
I - Introduction
Several celestial objects, including UV rich regions of planetary and reflection
nebulae, stars, HII regions and extragalactic sources, are characterized by the
unidentified infrared emission bands (UIR bands).-hThis family consists of features
at 3050 cm -1 (3.3p.m), 2950 cm'l(3.41_m), 16,/5 cm-l(6.21-tm), 1310 cm -1
(7.7p.m), 1150 cm -1 (8.6p.m) and 85 cm q (!1=,_m), with weaker signatures at
1785 cm q and 1430 cm "1, and was first detected in the planetary nebula NGC
7027 (Gillet et al., 1973; Merrill et al., 1975; Russel et al., 1977). The features
have typical widths of 3% to 10% of their wavelength (Allamandola, 1984;
Willner, 1984).
A few years ago, it was proposed that polycyclic aromatic hydrocarbon
species (PAHs) are responsible for most of the UIR bands (Duley and Williams,
1981; Leger and Puget, 1984; Allamandola, Tielens and Barker, 1985). This
hypothesis is based on a spectroscopic analysis of the observed features. The
3050 cm -1 band for example is known to be characteristic of the aromatic
stretching vibration in PAHs (Bellamy, 1968), as is the 6.2 i.tm signature.
Comparisons of observed infrared spectra with laboratory absorption spectra
of PAHs support the PAH hypothesis.(L_ger and d' Hendecourt, 1987;
Allamandola, Tielens and Barker, 1987). An example spectrum is represented in
Fig 1, where the Orion Bar 3.3 micron spectrum is compared with the absorption
frequencies of the PAHs Chrysene (C18H12), Pyrene (C16Hlo) and Coronene
(C24H12)- _'_'
This paper presents the laser-excited 3.3 _.m emission spectrum from a gas
phase PAH (azulene, CloH8). The in_frared fluorescence theory (IRF) is briefly
explained, followed by a description of the experimental apparatus, a report of
the results and discussion.
151
https://ntrs.nasa.gov/search.jsp?R=19910005621 2020-03-19T19:34:39+00:00Z
FREQUENCY Icm "I)
3100 3000 2900 2800
30__'_0 R
20 ION
0
le) PYRENE
i I t I I I
,l, I , @
CDRONENE
{g)
I i I I • I I i I
3.2 3.3 3.4 3.5 3.8
WAVELENGTH (_mt
I i
Fig 1:The 3 micron emission
spectrum from the Orion Bar
compared with the absorption
spectra of chrysene, pyrene and
coronene.
2 - Infrared Fluorescence Theory
The IRF emission due to a Av = 1 vibrational transition in a polyatomic molecule
of total vibrational energy E can be calculated from the basic expression for a
single oscillator (Herzberg, 1968; Durana and MacDonald, 1976; Rossi et al.,
1983)
I(E,i,v) = Nv(E) hvi Aiv,v'l
where vi is the frequency of the emitting mode, Aiv,v-1 is the Einstein coefficient
for the vm> v-1 transition (equal to v Ai 1,0 in the harmonic oscillator
approximation), and Nv(E) is the number of molecules with total energy E and v
quanta in the ith vibrational mode.
152
Under the ergodic assumption, the energy is distributed statistically among the
accessible vibrational states of the molecule, and
Ps-t (E - vhvi )
Nv(E ) = N(E)
Ps(E)
where N(E) is the total number of excited molecules, s is the number of
oscillators, ps(E) is the total density of states with vibrational energy E, and
ps-l(E-hvi) is the density of states, omitting the emitting mode i and its
vibrational energy content. Hence the theoretical IRF intensity for a particular
&v=l transition and for the emitting mode i
(i) I(E,i,v) = N(E) hvi v Ai 1,0
Ps-I(E - vhvi)
ps(E)
The reliability of the IRF theory recently has been tested by experimental data
that showed excellent agreement with theoretical predictions (Shi, Bernfeld and
Barker, 1988).
Emission spectra from PAHs can be predicted using equation 1 and an assumed
Lorentzian profile (30 cm -1 width) as shown in Fig 2 where the calculated
emission spectrum is for a C-H stretch mode (3050 cm -1) of chrysene as a
function of the vibrational energy content of the molecule (Barker,
Allamandola, and Tielens, 1987). The main peak corresponds to the transition
between the vibrational states v--1 and v=0. The second peak represents the
transition v=2---)1; the shift is due to the anharmonicity of the vibrational
potential of the molecule. A more realistic simulation would include the other C-H
stretch modes, which may have slightly different frequencies. All PAH species
would exhibit a similar spectrum, but the peak locations are peculiar to each
molecule and will be somewhat shifted.
FREQUENCY, _._-I
3200 3OOO 28OO
i 1 i b i
V I fir ATIONAL CALCU L.A'r'IED
t 4 _ ENERGY /,_ v == 1-_ EMISSION
>" CONTENT J_ SPECTRUM
Z 3 80.O00cm "1 I.ljl,_
ul,II
I,-,,
z eo.ooo
=.  .ooo ,,11 • - z-, ',
20.0oo--)/./\\\ A
' "'-..El \\\ );.\ ,=3-2
3.1 3.2 3.3 3.4 3.5 3.6
WAVELENGTH.
Fig 2 : .The calculated emission
spectrum for chrysene in the C-H
stretching region as a function of
vibrational energy content.
153
3 - Experimental
Azulene (CloH8) is a very convenient laboratory subject because its rapid
interconversion (IC) processes can be exploited in order to prepare vibrationally
excited molecules in the electronic ground state. In this experiment, a 308 nm
(32000 cm q) UV laser is used to excite azulene to its $2 excited electronic state
and subsequent IC to high vibrational levels of the electronic ground state takes
place in ~1.4 ns, a lifetime much shorter than the interval between collisions.
This method thus provides an ensemble of vibrationally excited molecules which
emit infrared light via fluorescence.
The present experiment employs both IRF and optoacoustic techniques (Shi,
Bernfeld and Barker, 1988). The first method measures the IRF emission intensity
while the second one determines the number of vibrationally excited molecules.
The IRF emission per azulene molecule can be deduced from the ratio of these two
measurements.
The experimental apparatus is presented schematically in Fig 3. ]'he
fluorescence cell is fitted with Suprasil fused silica windows for the laser beam.
The fluorescence is viewed through a sapphire window by a 77 K InSb
photovoltaic detector (5 p.s time-response) and matched preamplifier. A spherical
gold mirror with a 5 cm focal length is placed inside the cell so that the detector
matches the mirror center of curvature. The detector image is then located on the
detector itself, which views a 77 K background. This method improves the
fluorescence signal by collecting more light and by reducing the background noise.
The azulene vapor pressure in both cells is regulated by maintaining the azulene
reservoirs at fixed temperature with a temperature-regulated bath
]'he fluorescence emisson is isolated by a 2.5-5 p.m bandpass filter (BPF) and
wavelength resolved by a CVF filter maintained at 77 K. The resolution of the
system is defined by an ajustable slit placed between the BPF and the CVF. A
40 cm -1 resolution was chosen in this experiment. The sensitivity of the
instrument is essentially constant over the wavelength range of interest. Both
the IRF emission and the optoacoustic signal are simultaneously acquired and
averaged using a digital oscilloscope. The data are transfered to a computer and
analysed using non-linear least squares.
Laser Mirror
11 o
,, 11BPF =::==
Slit -- --
CVF ="=_"_
Detector [_]Pre.Arnpl.
Fig 3 • Experimental Apparatus
154
I ° ]
E: Microphone
Amplifier
Signal averager
Computer
4- Results and discussion
A typical fluorescence decay signal is shown in Fig 4. The decay is due to
collisional deactivation and is fitted by an exponential function. The IRF intensity
at t=0 is then extrapolated from the intensity at t= 5_s, the time response of the
system.
A preliminary IRF emission spectrum is presented in Fig 5. This spectrum
resembles the observed IR spectrum, although azulene, used in this experiment
for its photophysical properties, is probably not responsible for the IR emission
in the interstellar medium, where a typical PAH size would range between
20 and 50 carbon atoms ( Allamandola, Tielens and Barker, 1987 ). The peak
position in Fig 5 is 3075 __.20 cm -1, which corresponds to the v=l--->0 C-H
stretching transition frequency. The measured spectrum is for all azulene C-H
stretch modes, which have frequencies between 3020 cm -1 and 3080 cm-l(Chao
and Khanna, 1977). The asymmetry in the spectrum may result from the
convolution of the ,_v=l-0 and AV=2-1 with the bandpass of the system, which is
about 40 cm °1. Thus the v=2---_ 1 transition can not be resolved under these
conditions. Nevertheless, the preliminary results presented in this paper are
consitent with the hypothesis that PAHs are responsible for the UIR bands
emission.
Experiments now underway are using a higher resolution (28 cm -1) and should
provide a better-resolved IR fluorescence spectrum from azulene. Future work
will investigate other PAHs, bah in cells and in cold molecular beam expansions,
which mimic temperatures found in the interstellar medium.
0.00
>" -0.01
tn
r-
<D
c"
-- -0.02
°
-0.03 ,
-50 0
, ! 1
50 1 00
Time (gsec)
• I
150 2OO
Fig 4 • IRF decay curve
for azulene.
155
¢-
¢}
8
o.
O
_=
LL
rr"
1.1
1.0
0.9
0.8
0.7
0.6
0.5-
0.4-
0.3-
0.2'
0.1
0.0
3.0
t
I
I
i I
I
iI
iI
IIIlII :
I " I I I ! " 1
3.1 3.2 3.3 3.4 3.5 3.6 3.7
Wavelength (p.m)
Fig 5 • IRF emission spectrum
for azulene
Acknowledgements • This work is funded, in part, by the National Science
Fundation, Gallactic Astronomy Program.
References •
Allamandola,
Kessler,
Allamandola,
Allamandola,
eds. D.J.
L.J.: 1984, in Galactic and Extragalactic Infrared Spectroscopy, eds. M.F
J.P. Phillips, Reidel, Dordrecht
L.J., Tielens, A.G.G.M and Barker, J.R • 1985, Ap. J. Letters 290, L25
L.J., Tielens, A.G.G.M and Barker, J.R.: 1987, in Interstellar Process,
Hollenbach and H.A. Thronson, Jr, Reidel, Dordrecht
Barker, J.R., Allamandola, L.J. and Tielens, A.G.G.M.: 1987, Ap. J. Letters 315, L61
Bellamy, L.J., 1958, The Infrared Spectra of Complex organic Molecules, eds. J.
Wiley and Sons, New York
Chao R.S and Khanna, R.K: 1977, Spectroschimica Acta 33A, 53
Durana, J.F. and MacDonald, J.D.: 1977, J. Chem. Phys. 64, 2518
Duley, W.W. and Williams, D.A.: 1981, M.N.R.A.S, 196, 269
Gillett, F.C., Forrest, W.J. and Merrill, K.M.: 1973, Ap. J. 183, 87
d' Hendecourt L.B. and L_ger, A.: 1987, in Planetary and Proto-Planetary Nebulae :
from IRAS to ISO, 203-220, eds. Preite martinez, A., Reidel
Herzberg, G.H.: 1968, Infrared and raman Spectra of plyatomic molecules, eds. D.
van Nostrand Co., Princeton
L_ger, A. and Puget, J.L.: 1984, Astronomy and Astrophysics 137, L5
Merrill, K.M., Soifer, B.T. and Russel, R!W.: 1975, Ap. J. Letters 200, L37
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156


Infrared fluorescence from PAHs in the laboratory

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