HIGH PRECISION MID-IR SPECTROSCOPY OF N2O NEAR 4.5 μm Wei-jo (Vivian) Ting and Jow-Tsong Shy Department of Physics National Tsing Hua University Hsinchu, Taiwan

 Similar to CO 2, N 2 O is one of the important greenhouse gases.  No extensive heterodyne frequency measurements of the line center.  New and refined molecular constants are of great importance to atmospheric chemistry, meteorology, and astrophysics. Motivation N2ON2O CO 2

Partial energy diagram of N2O cm cm cm cm cm cm cm cm -1 Green : Laser transitions that have been measured by Whitford et al.(1975) Blue : Suggested by Dr. A. G. Maki. Red: Can be derived from Blue and Green transitions.

mW PPLN Difference Frequency Generation Source Ti:Sapphire laser Nd:YAG laser MgO:PPLN Temperature stability < 0.05 ℃ Ge plate 1 W tunable: 700 ~1000 nm 8 W through fiber nm DFG radiation ~1 4.5 μm 45 mm long

Experimental Set-up CaF 2 window InSb detector DFG N 2 o cell Lock-in amplifier Ti:sa laser Locking point

Frequency calibration Ti:sapphire laser (f TiS ) Optical Frequency Comb Nd:YAG laser (f YAG ) Iodine hyperfine transition f TiS - f YAG =f DFG DFG absolute frequency

Uncertainty  OFC 5 kHz  Iodine stabilized of Nd:YAG laser 5 kHz  N 2 O stabilized Ti:sapphire laser 25 kHz Uncertainty 25 kHz

Saturation spectroscopy of N2O R(10) Gas pressure ~2 mTorr DFG power ~ 1 mW Modulation Frequency: 23 kHz Modulation width: 2.0 MHz S/N ratio: bandwidth R(10) 3 rd derivative spectrum

Signal Optimization Maximum signal Near 2.5 mTorr The changes of 3 rd derivative signal with different gas pressure

Linewidth analysis FWHM :2.372 ±0.062 MHz The peak amplitude of 3 rd derivative signal with different modulation depth. Fitting function: h(δA) =( P1 δA +P2 δA ² + P3 δA ³ )/( P4+P5 δA +P6 δA ² + P7 δA ³ ). Simulate by H.M. Fang Ref: Nakazawa (1986) δA=2W/δL W: Modulation Width

Measurements of R(10) R(10): Mean frequency = 66,929,219,708 kHz, STD = 1.6 kHz

Observed Transitions J Observed frequency (MHz) HITRAN04 frequency (MHz) Difference (MHz) transitions have been measured. Their difference with HITRAN04 data is ≤ 1 MHz. Reference of frequency data in HITRAN04: R.A. Toth, J. Opt. Soc. Am. B 4, (1987).

Molecular Constants (1) Fitting formula: F(J) is rotational energy F v (J) = B v J(J +1)−D v J 2 (J +1) 2 +H v J 3 (J +1) 3 +· · · ConstantsToth (1987)This Work (Combined with Toth’s data) ν0ν (16) (46) B(00 0 1) (45) (16) D(00 0 1) × (60) (94) H(00 0 1) × (850) (106) B(00 0 0) a D(00 0 0) ×10 7 a H(00 0 0) ×10 13 a a. R.A. Toth, J. Opt. Soc. Am. B 3, (1986).

Molecular Constants (2) Transition Measured Frequency (MHz) Prediction from refined molecular constants Prediction from molecular constants by Toth (1987) Frequency (MHz) Difference (MHz) Frequency (MHz) Difference (MHz) R(5) (28) R(6) (28 ) R(7) (30) R(8) (28) R(9) (28) R(10) (22) R(11) (27) R(12) (27) R(14) (33) R(15) (30) R(16) (23) R(20) (24) R(24) (25) R(30) (25) R(35) (26) R(40) (27) R(45) (28) One order of magnitude improvement.

 17 R-branch transitions of the fundamental band have been measured to an accuracy of 25 kHz.  Refine the molecular constants of vibrational levels. Summary

 Fundamental band high J (J > 45) R-branch transitions. P-branch transitions  Hot band (01 1 1← ) transitions  ← 0000 band transitions Future Works

Ching-Hsiang Hsieh for frequency measurements $$ National Science Council & Ministry of Education, Taiwan Acknowledgements