Class 3 The PAH Spectrum, what does it tell us??
PAH Vibrations! CH str! CC str! CC str /CH ip! CH oop! 3! 4! 5! 6! 7! 8! 9! 10! 15! Wavelength (µm)! NASA Ames! Astrochemisty Lab! 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 (N C > 50 C atoms) can accommodate Class C emission component Class A component anomalous A" B" C" Peeters et al. 2002 A&A, 360, 1089. Composite" spectra of PAH ions"
Polycyclic Aromatic Nitrogen Heterocycles - PANHs Exhibit 1600 IR spectroscopic cm -1 CC stretching characteristics features very that are similar very to similar those to those of the of parent the parent PAH. PAH PANH Position of strongest 1500-1600 cm -1 cation band (parent PAH) PANH Position of strongest 1500-1600 cm -1 cation band (parent PAH) N 1549 cm -1, 6.456 µm (1553 cm -1, 6.439 µm) N 1538, cm -1, 6.502 µm (1540 cm -1, 6.494 µm) N 1559 cm -1, 6.414 µm (1560 cm -1, 6.410 µm) N 1531 cm -1, 6.532 µm (1540 cm -1, 6.494 µm) N 1553 cm -1, 6.439 µm (1560 cm -1, 6.410 µm) N 1568 cm -1, 6.378 µm 1585 cm -1, 6.309 µm N 1564 cm -1, 6.394 µm (1560 cm -1, 6.410 µm) N 1574 cm -1, 6.353 µm (1590 cm -1, 6.289 µm) Chem. & Phys. Matrix Isolated Species 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
N Substituted coronene cations 6.10 Wavelength (µm) 6.20 6.30 6.40 6.50 6.60 Theoretical Calculations - C. Bauschlicher Normalized Absorbance N1b-Coronene Cation, C 23 H 11 N + 1650 1625 1600 1575 1550 1525 1500 Wavenumber (cm -1 ) Chem. & Phys. Matrix Isolated Species Coronene Cation, C 24 H 12 + 7
N Substituted coronene cations 6.10 Wavelength (µm) 6.20 6.30 6.40 6.50 6.60 Theoretical Calculations - C. Bauschlicher Normalized Absorbance N3b-Coronene Cation, C 23 H 11 N + N2b-Coronene Cation, C 23 H 11 N + N1b-Coronene Cation, C 23 H 11 N + 1650 1625 1600 1575 1550 1525 1500 Wavenumber (cm -1 ) Chem. & Phys. Matrix Isolated Species Coronene Cation, C 24 H 12 + 8
Endoskeletal PANHs and the 6.2 µm Emission Band" Normalized Absorbance 6.10 Wavelength (µm) 6.20 6.30 6.40 6.50 6.60 5N-Circumcor N19 Cation + 4N-Circumcor N21 Cation + 3N-Circumcor N29 Cation + 2N-Circumcor N35 Cation + 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." 1650 + Circumcoronene Cation, C 54 CH 5418 H + 18 1625 1600 1575 1550 1525 1500 Wavenumber (cm -1 ) 1 N-Circumcor N23 Cation + 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 " Dipole Moments (Debye) Dipole Moments (Debye) Species µ a µ b µ Species µ a µ b µ N-coronene cations N-circumcoronene cations 1N 5.48 0.19 5.49 1N 9.23 0.23 9.23 2N 3.69 0.00 3.69 1'N 6.99 0.00 6.99 3N 2.67 0.00 2.67 2N 6.77 0.47 6.79 3N 5.30 1.20 5.43 N-ovalene cations 4N 4.55 0.00 4.55 1N 7.10 0.98 7.17 5N 1.32 0.00 1.32 1'N 5.38 4.81 7.21 1''N 4.92 4.26 6.51 N-circum-circumcoronene cations 1'''N 0.00 3.47 3.47 2N 10.12 0.33 10.13 2N 5.25 1.19 5.38 2'N 9.09 0.00 9.09 2'N 1.59 3.65 3.98 3N 7.47 1.94 7.72 3N 4.32 1.02 4.44 3'N 8.31 0.00 8.31 3'N 1.29 1.99 2.37 4N 7.33 0.63 7.72 4N 0.00 1.56 1.56 5N 4.75 0.62 4.79 6N 3.06 0.00 3.06 7N 2.54 0.00 2.54 Hudgins, Bauschlicher, & Allamandola (2005)! 11
and rotational constants in the" 0.34-0.01 GHz (340 10 MHz) range " Rotational Constants (GHz) Species R a R b R c N-coronenes 0.334-0.337 0.331-0.336 0.166-0.168 N-ovalenes 0.238 0.148 0.091 N-circumcoronenes 0.066 0.066 0.033 N-circum-circumcoronenes 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 10 5 Model Ancillary data WMAP Planck Flux density [Jy] 10 4 10 3 10 2 Planck Data 10 1 10 0 1 10 100 1000 Frequency [GHz] The AME (10 to 100 MHz) falls precisely in the range expected from large PANHs (and PAHs)!!!
PAH Vibrations! CH str! CC str! CC str /CH ip! CH oop! 3! 4! 5! 6! 7! 8! 9! 10! 15! Wavelength (µm)! NASA Ames! Astrochemisty Lab! Vibration - S. Langhoff! Orion ISO Spectrum - E. Peeters!
End part 1 Class 3 Go to Christiaan s work