A laboratory search for the rotational spectrum of C3H+

Kyle N. Crabtree, Oscar Martinez Jr., Carl A. Gottlieb, Michael C. McCarthy

Horsehead Nebula in IR from Hubble, APOD April 22, 2013

Hydrocarbon chemistry

K. Didriche and M. Herman (2010) Chem. Phys. Lett., 496, 1–7P. B. Rimmer et al., (2012) A&A, 537, A7

ISM

Titan

Observations of hydrocarbons in Horsehead

Nebula

J. Pety et al. 2012, A&A, 548, A68

PDRCore

WHISPER ProjectIRAM 30m

Observations of hydrocarbons in Horsehead

Nebula

J. Pety et al. 2012, A&A, 548, A68

PDRCore

WHISPER ProjectIRAM 30m

Spectroscopic evidence

� Observations consistent with:

� Linear, nearly rigid rotor (Beff = 11244.95 MHz)

� Closed-shell electronic state

� 3 heavy (second-row) atoms

� PDR region contains C3H2, C2H, etc.

� Assigned to l-C3H+; no lab

data availableRigid Rotor Spectrum

C3H+ properties and chemistry

H C C C

C

C

CH

C+

C2H2

C3H+

C3H2+ C3H3

+

C3H C3H2

HC3N C3

C+ H3+

e-

C2H

e-

e-e-

H2 H2 ∆E

17 kcal/mol5950 cm-1

8550 K0.737 eV

Present chemical models cannot account for C3H+, C2H, and C3H2 simultaneously!

S. Ikuta (1997) J. Chem. Phys., 106, 4536

Not so fast! Is it really l-C3H+?

� Assignment to l-C3H+ called into question on basis of

state-of-the-art quantum chemical calculations

� 2 possibilities (assignment or calculation is wrong!):

� Molecule is actually slightly bent (not C3H+!)

� Calculations fail to capture large amplitude vibrational motion in C3H

+

Constant Observed Calculated |% Difference| Typ. |% Diff.|

B (MHz) 11244.9512 11262.68 0.17 0.1—0.2

D (kHz) 7.766 4.248 45.3 5—10

H (mHz) 560 0.375 99.9 -----

E(J) = BJ(J+1) – DJ2(J+1)2 – HJ3(J+1)3

X. Huang et al. (2013) ApJL 768, L25

Lab search: FTMW spectroscopy

� Fourier Transform Microwave spectroscopy

� Highly-sensitive technique for rotational spectroscopy over 5-43 GHz spectral range

� Detection of reactive species (CnH

-, CnH, HNNO+, NNOH+, many more)

Detection of putative 1-0 line

0.05% C2H2:Heor

0.05% C4H2:He

-1 kV

fmeas

2-1 transition: double resonance

fDR

fFTM

J = 0

J = 2

J = 1fFTM

fDR

22489.86 MHz

44979.56 MHz

What does it mean?

� Lab and space carriers are the same molecule: linear (or very nearly linear) with B = 12144.95 MHz

� Molecule is a hydrocarbon; contains 3 C atoms and at least 1 H

� Molecule has closed shell electronic state

� Molecule is an ion, so has odd number of H atoms

If it’s not C3H+, then what could it be?

� C3H- possesses nearly-

linear 1A state

� Large barrier: cyclic to linear conversion

� Could C3H- be the carrier

of the astronomical lines?

Species B D

Astronomical 11244.95 7.766

C3H+ (calc) 11262.68 4.428

C3H- (calc) 11213.51 8.795

Lakin et al (2001) J. Chem. Phys. 115, 3664; Huang et al. (2013), ApJL, in press; T. J. Lee, personal communication

Millimeterwave absorption spectroscopy

� 3 m long, liquid N2-cooled discharge tube

� Frequency range: ~80-300 GHz

� Ion charge determination:

HV–

Cations (+)

Anions (–)

– +

Anions (–)

Cations (+)

Millimeterwave absorption spectroscopy

� 3 m long, liquid N2-cooled discharge tube

� Frequency range: ~80-300 GHz

� Ion charge determination:

HV+ –

Cations (+)

Anions (–)

FTMW survey near C3H+ transition

C3H+

(too weak)

C3H2Σu, ν4 = 1(too weak)

Most lines in survey are 1-0 transitions of 2Σ molecules with B ~11240 MHz! (C3H)

None have been previously reported

Unexpected discovery: new C3H Σ states

� C3H is known interstellar molecule

� 2 low-energy bending modes (ν4 and ν5)

� Previous lab measurements found pure rotational transitions from ν4 = 1, 2Σu

state.

� No pure-rotational lines from vib. excited state detected in space (cf CCH)

Caris et al. (2009) J. Mol. Spectrosc. 253, 99

Vibrationally excited C3H

To do:Millimeterwave spectroscopyQuantum calclationsAssignmentsAstronomical relevance?

Summary

H C C C

orH

C C C

+

–PDRCore