1. 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

2.  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

3. Partial energy diagram of N2O 00 0 0 00 0 1 02 0 0 01 1 0 10 0 0 01 1 1 2223 cm -1 589 cm -1 2789 cm -1 2209 cm -1 1285 cm -1 1168 cm -1 939 cm -1 1056 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.

4. 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 amplifier @1064 nm DFG radiation ~1 mW @ 4.5 μm 45 mm long

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

6. 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

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

8. 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: 450 @1Hz bandwidth R(10) 3 rd derivative spectrum

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

10. 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

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

12. Observed Transitions J Observed frequency (MHz) HITRAN04 frequency (MHz) Difference (MHz) 566812939.20266812940.3641.162 666836611.18066836611.9760.797 766860075.34966860076.7321.383 866883331.58566883331.6330.048 966906379.73466906379.677-0.057 1066929219.70866929220.8651.156 1166951851.37466951852.1970.823 1266974274.60166974273.675-0.926 1467018495.33367018496.0610.727 1567040292.48967040293.9701.481 1667061880.80267061882.0251.223 2067146142.49367146141.693-0.800 2467227052.08967227052.6800.591 3067342115.21667342116.0230.808 3567432209.30267432209.6530.351 4067517023.23467517023.9370.702 4567596543.41567596543.8860.472 17 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, 357-374 (1987).

13. 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) ν0ν0 2223.756764 (16)2223.75674420 (46) B(00 0 1) 0.415559510 (45)0.4155595032 (16) D(00 0 1) ×10 7 1.754675 (60)1.754676 2 (94) H(00 0 1) ×10 13 -0.13626 (850)-0.13540 (106) B(00 0 0) a 0.419011001 D(00 0 0) ×10 7 a 1.7609193 H(00 0 0) ×10 13 a -0.16529 a. R.A. Toth, J. Opt. Soc. Am. B 3,1263-1281 (1986).

14. 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)66812939.202 (28)66812939.225-0.02366812939.824-0.622 R(6)66836611.180 (28 )66836611.227-0.04766836611.826-0.647 R(7)66860075.349 (30)66860075.383-0.03466860075.983-0.634 R(8)66883331.585 (28)66883331.603-0.01966883332.203-0.618 R(9)66906379.734 (28)66906379.767-0.03366906380.367-0.633 R(10)66929219.708 (22)66929219.725-0.01766929220.355-0.647 R(11)66951851.374 (27)66951851.388-0.01466951851.987-0.613 R(12)66974274.601 (27)66974274.604-0.00466974275.234-0.633 R(14)67018495.333 (33)67018495.2510.08267018495.911-0.577 R(15)67040292.489 (30)67040292.4710.01867040293.131-0.642 R(16)67061880.802 (23)67061880.7660.03667061881.426-0.624 R(20)67146142.493 (24)67146142.4130.08067146143.102-0.609 R(24)67227052.089 (25)67227052.0500.03967227052.770-0.681 R(30)67342115.216 (25)67342115.1540.06267342115.963-0.748 R(35)67432209.302 (26)67432209.2630.03967432210.132-0.830 R(40)67517023.234 (27)67517023.247-0.01367517024.177-0.942 R(45)67596543.415 (28)67596543.467-0.05267596544.486-1.071 One order of magnitude improvement.

15.  17 R-branch transitions of the fundamental band have been measured to an accuracy of 25 kHz.  Refine the molecular constants of 00 0 1 vibrational levels. Summary

16.  Fundamental band high J (J > 45) R-branch transitions. P-branch transitions  Hot band (01 1 1← 01 1 0) transitions  01 1 1 ← 0000 band transitions Future Works

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