Submillimeter-wave Spectroscopy of [HCOOCH 3 ] and [H 13 COOCH 3 ] in the Torsional Excited States Atsuko Maeda, Frank C. De Lucia, and Eric Herbst Department of Physics, The Ohio State University Columbus, OH, WK03 63 rd International Symposium on Molecular Spectroscopy June 18 th, 2008

A 0.60 cm -1 (18 GHz) E cm -1 (0.4 GHz) A E 6.1 cm -1 (183 GHz) A Senent, Villa, Meléndez and Dominguez-Gomez, ApJ. 627, 567 (2005) CH 3 Internal Rotation in HCOOCH 3 Potential curve with zero-point vibrational correction Numbers of studies! A few studies Maeda et al. ApJS, 175 (2008)

HCOOCH 3 in Interstellar Media — One of abundant interstellar molecules in hot cores & corinos, e. g., Orion KL, Sgr B2… at Orion KL Hot Core Ultra-compact HII region O,B star T = K n = 10(6)-10(8) cm -3 Cold background ~20 U-lines have been identified to methyl formate in v t =1 Kobayashi et al. ApJ, 657, L17 (2007)

FASSST spectrometer * GHz with three BWOs *100 scans accumulation *up & down-ward sweeps → averaged Frequency calibration with SO 2 lines + ring cavity Commercial HCOOCH 3 & H 13 COOCH 3 (Cambridge Isotope Laboratories, Inc.) A few mTorr in 8 m-long aluminum cell under room temperature Measurement

Satellite Lines of HCOOCH 3 with CAAARS * Computer Aided Assignment of Asymmetric Rotor Spectra; Medvedev et al. J. Mol. Struct. 742, 299 (2005) Methyl formate in v t = 0 (A, E) Series of transitions with K a = 0, 1 Satellite lines with doublets 60 % of intensity of line → v t = 1 Satellite lines with doublets 40 % of intensity of line → v t = 2 AE E AE

Reduced Fortrat Diagram of a-type R Transitions of HCOOCH 3 in v t = 0,1,2 500 lines K a ≤ 9 in A K a ≤ 3 in E 1600 lines K a ≤ 23 in A & E Previous Microwave studies

Effective Rotational Hamiltonian for Asymmetric Molecules with One or Two Internal Rotors by Groner * based on principal axes system H rot for asymmetric rotor Torsional energy Coriolis interaction negligible where * Peter Groner, J. Chem. Phys. 107, 4483 (1997)

Molecular Constants of HCOOCH 3 [MHz] 28 of 35 parameters for v t = 1 (A & E) (~1200 lines w/ J ≤ 15, K a ≤ 9) with  = of 27 parameters for v t = 2 (A & E) (~200 lines w/ J ≤ 15, K a ≤ 4) with  = 0.81 Parameters for asymmetric rotors Parameters for torsional energy & tunneling terms Comparable

Molecular Constants of HCOOCH 3 [MHz] Orders of magnitude larger ! Parameters for torsional energy & tunneling terms Parameters for asymmetric rotors

Intensity Ratios & Energy Differences ΔE = 127 cm -1 ΔE = 235 cm -1 Theoretical v t = 1 (A); cm -1 v t = 2 (A); cm -1 Comparable! a-type R transitions v t = 0 (A)  v t = 2 (A) a-type R transitions v t = 0 (A)  v t = 1 (A)

Reduced Fortrat Diagram of a-type R transitions of H 13 COOCH 3 in v t = 0,1, lines K a ≤ 22 in v t =1 547 lines K a ≤ 10 in v t =2 (A) K a ≤ 3 in v t =2 (E)

Molecular Constants of H 13 COOCH 3 28 of 35 parameters for v t = 1 (A & E) (~910 lines w/ J ≤ 15, K a ≤ 9) with  = of 27 parameters for v t = 2 (A & E) (~200 lines w/ J ≤ 15, K a ≤ 4) with  = 1.07

Residuals (obs.-calc. frequencies) J > 15 K a > 9 v t = 1 v t = 2 J > 15 K a > 4 Omitted in the final fits Showing Large & Systematic Deviations (60 % of assigned transitions) (40 % of assigned transitions) v t = 1

Perturbations at High J and High K a v t =0 v t =1 v t =2 1) Large deviations at J > 15. 2) Severe perturbation at K a > 9 in v t = 1 and K a > 4 in v t = 2. 3) Missing E at low K a in v t = 2. Interactions among different torsional states? Fermi interaction with C-O-C deformation (318 cm -1 )? Close to barrier (370 cm -1 )?

Summary Submillimeter-wave lines in GHz were newly assigned to HCOOCH 3 and H 13 COOCH 3 in v t = 1 & 2. Torsional assignments were supported with reduced-Fortrat-diagram & energy differences deduced from relative intensity comparison. Transitions with J ≤ 15 & K a ≤ 9 in v t = 1, and transitions with J ≤ 15 & K a ≤ 4 in v t = 2 were fitted within ~100 kHz. Interactions among different torsional states? A global fit with diagonal terms?? Or, interactions with other vibrational states? Acknowledgement Peter Groner for his suggestions on analysis with ErHam NASA for its support ARO for its support