Class 3 The PAH Spectrum, what does
it tell us??
PAH Vibrations!
CCstr!
CHstr!
3!
4!
NASA Ames!
Astrochemisty Lab!
5!
CCstr/CHip!
CHoop!
6!
7! 8! 9! 10!
Wavelength (µm)!
15!
Vibration - S. Langhoff!
Orion ISO Spectrum - E. Peeters!
But the Real Treasure (and some
nice surprises) are in the Details...
•  The global similarity of the interstellar emission"
  single chemical family; molecule-sized polycyclic aromatic
compounds"
•  Differences at the most detailed levels"
  reflect spectroscopic characteristics of that portion of the
population that is changing in response to changing physical
and chemical conditions. "
•  Therefore, it is the details of the interstellar spectra
that hold the key to exploiting PAHs as probes of the
emitting regions. "
Slide adapted from Hudgins- IAU Symposium 231!
Analysis of the 6.2 µm Emission Band
Observations of the 6.2 µm emission feature have shown:
•  Range: 6.203µm - 6.299µm.
•  Composite of 2 bands, not a
continuous dist’n of 1.
•  Class A, B bands by far the
most common.
Emission at this position is
dominated by PAH ions
•  Large PAH cations (NC > 50
C atoms) can accommodate
Class C emission component
•  Class A component
anomalous
A"
B"
Peeters et al.
2002 A&A, 360, 1089.
Composite"
spectra of
PAH ions"
C"
Polycyclic Aromatic Nitrogen
Heterocycles - PANHs
Exhibit
Exhibit1600
IR spectroscopic
cm-1 CC stretching
characteristics
features very
that are
similar
veryto
similar
those
to those
of theofparent
the parent
PAH. PAH
PANH
N
N
Position of strongest 1500
- 1600 cm-1 cation band
(parent PAH)
PANH
N
1549 cm-1, 6.456 µm
(1540 cm-1, 6.494 µm)
N
1559 cm-1, 6.414 µm
(1560 cm-1, 6.410 µm)
1553 cm-1, 6.439 µm
N
(parent PAH)
1538, cm-1, 6.502 µm
(1553 cm-1, 6.439 µm)
N
Position of strongest 1500
- 1600 cm-1 cation band
1531 cm-1, 6.532 µm
(1540 cm-1, 6.494 µm)
N
1568 cm-1, 6.378 µm
(1560 cm-1, 6.410 µm)
1585 cm-1, 6.309 µm
1564 cm-1, 6.394 µm
1574 cm-1, 6.353 µm
(1560 cm-1, 6.410 µm)
N
Chem. & Phys. Matrix Isolated Species
(1590 cm-1, 6.289 µm)
5
Polycyclic Aromatic Nitrogen
Heterocycles - PANHs
Substitution at an edge site = “exoskeletal” PANH
Substitution at an internal site = “endoskeletal”
PANH
"
Chem. & Phys. Matrix Isolated Species
6
_
_
_
Theoretical Calculations - C. Bauschlicher
Normalized Absorbance
_
6.60
_
6.10
Wavelength (µm)
6.20 6.30 6.40 6.50
_
N Substituted coronene cations
N1b-Coronene Cation, C2 3H1 1N+
1650 1625 1600 1575 1550 1525 1500
-1
Wavenumber (cm )
Chem. & Phys. Matrix Isolated Species
Coronene Cation, C2 4H1 2+
7
_
_
_
_
6.60
_
6.10
Wavelength (µm)
6.20 6.30 6.40 6.50
_
N Substituted coronene cations
Theoretical Calculations - C. Bauschlicher
Normalized Absorbance
N3b-Coronene Cation, C2 3H1 1N+
N2b-Coronene Cation, C2 3H1 1N+
N1b-Coronene Cation, C2 3H1 1N+
1650 1625 1600 1575 1550 1525 1500
-1
Wavenumber (cm )
Chem. & Phys. Matrix Isolated Species
Coronene Cation, C2 4H1 2+
8
6.60
_
_
_
_
_
6.10
Wavelength (µm)
6.20 6.30 6.40 6.50
_
“Endoskeletal” PANHs and the 6.2 µm
Emission Band"
Normalized Absorbance
N19 Cation +
5N-Circumcor
N21 Cation +
4N-Circumcor
N29 Cation +
3N-Circumcor
N substitution within the carbon skeleton
of a PAH produces a depth-dependant
"blue shift in the"
"position of the"
"dominant CC"
"stretching"
"feature near "
"6.2 µm."
N35 Cation +
2N-Circumcor
+
N23 Cation
1’N-Circumcor
+ +
Circumcoronene Cation,
Circumcoronene
Cation,C5C4H541H
8 18
1650 1625 1600 1575 1550 1525 1500
-1
Wavenumber (cm )
The position of the!
nominal interstellar 6.2 µm
emission band may provide a
tracer of N in interstellar dust.
Chem. & Phys. Matrix Isolated Species
9
PANHs have a large permanent dipole (µ),
PAHs do not
N
Microwave observatories should be able to detect
the “finger prints” of PANHs in the interstellar
environment. This would allow the identification of individual
aromatic molecules in the interstellar medium.. Image Gallery-Radio Telescopes
http://www.jb.man.ac.uk/vlbi/images/telbig/dsn15.gif
Andy Mattioda
PANH cations all possess significant
dipole moments…"
Species
Dipole Moments
(Debye)
µa
µb
µ
N-coronene cations
1 N 5.48
2 N 3.69
3 N 2.67
0.19
0.00
0.00
5.49
3.69
2.67
N-ovalene cations
1N
1'N
1''N
1'''N
2N
2'N
3N
3'N
4N
0.98
4.81
4.26
3.47
1.19
3.65
1.02
1.99
1.56
7.17
7.21
6.51
3.47
5.38
3.98
4.44
2.37
1.56
7.10
5.38
4.92
0.00
5.25
1.59
4.32
1.29
0.00
Hudgins, Bauschlicher, & Allamandola (2005)!
Species
Dipole Moments
(Debye)
µa
µb
µ
N-circumcoronene cations
1 N 9.23
1 ' N 6.99
2 N 6.77
3 N 5.30
4 N 4.55
5 N 1.32
0.23
0.00
0.47
1.20
0.00
0.00
N-circum-circumcoronene cations
2 N 10.1 2 0.33
2 ' N 9.09 0.00
3 N 7.47 1.94
3 ' N 8.31 0.00
4 N 7.33 0.63
5 N 4.75 0.62
6 N 3.06 0.00
7 N 2.54 0.00
9.23
6.99
6.79
5.43
4.55
1.32
10.1 3
9.09
7.72
8.31
7.72
4.79
3.06
2.54
11
…and rotational constants in the"
0.34 - 0.01 GHz (340 – 10 MHz) range"
Specie s
N-coronenes
N-ovalenes
N-circumcoronenes
N-circum-circumcoronene s
Rotational Constants (GHz )
Ra
Rb
Rc
0.334 - 0.337
0.331 - 0.336
0.166 - 0.168
0.238
0.148
0.091
0.066
0.066
0.033
0.021
0.021
0.011
Hudgins, Bauschlicher, & Allamandola (2005)!
•  Interstellar PANHs should produce a dense forest of lines over a very
broad spectral range."
•  Interstellar PANHs may contribute to anomalous galactic background
emission at radio wavelengths. "
•  Interstellar PANHs may represent an interesting, albeit challenging,
subject for an interstellar search."
Chem. & Phys. Matrix Isolated Species
12
The Anomalous Microwave Emission (AME)
on the Cosmic Microwave Background Flux density [Jy]
105
Model
Ancillary data
WMAP
Planck
104
Planck
Data
103
102
101
100
1
10
100
Frequency [GHz]
1000
The AME (10 to 100 MHz) falls precisely in the range
expected from large PANHs (and PAHs)!!!
PAH Vibrations!
CCstr!
CHstr!
3!
4!
NASA Ames!
Astrochemisty Lab!
5!
CCstr/CHip!
CHoop!
6!
7! 8! 9! 10!
Wavelength (µm)!
15!
Vibration - S. Langhoff!
Orion ISO Spectrum - E. Peeters!
End part 1 Class 3
Go to Christiaan’s work