Fukuoka Univ. A. Nishiyama, A. Matsuba, M. Misono Doppler-Free Two-Photon Absorption Spectroscopy of Naphthalene Assisted by an Optical Frequency Comb 1

Introduction ～ High Resolution Spectroscopy High Resolution and High Precision S1S1 S0S0 S2S2 T1T1 T2T2 Vibrational structure Rotational structure → Basic properties → Chemical reaction processes Detailed Structures and Dynamics in excited states of molecules 2

Introduction ～ Conventional Spectroscopic Systems EOM confocal etalon Etalon marker … Uncertainty ～ a few MHz Frequency scanning cw Laser Frequency scanning cw Laser Frequency High resolution spectroscopy … Resolution ～ linewidth of cw laser ・ Doppler-free two-photon absorption spectroscopy ‧ Supersonic jet ～ Ideal high resolution spectroscopic system ～ Resolution and Uncertainty < natural width ( ～ 1 MHz) 3

Absolute frequency f n = nf rep + f CEO （ n : mode number ） f CEO (Carrier-Envelope Offset) f rep (Repetition rate ) fnfn → Precise optical frequency ruler Spectral Intensity Optical Frequency Introduction ～ Optical Frequency Comb GPS clock ～ Comb stabilized to GPS clock … Uncertainty = a few kHz 4

● Development of High Resolution Spectroscopic System Frequency measurement system with an optical frequency comb … High Precision < natural width ( ～ 1 MHz) Doppler-free two-photon absorption spectroscopy … High Resolution < natural width ( ～ 1 MHz) ● Measurement of detailed structure of naphthalene spectra in whole molecular band →Dynamics in excited states Aim of Our Study 5

Doppler-free Two Photon Absorption Spectroscopy Ti:S Comb Power : 400 mW nm Ti:S Comb Power : 400 mW nm PC λ/4 λ/2 PBS PCF Single mode cw dye Laser Power : 1300 mW Linewidth: 100 kHz Single mode cw dye Laser Power : 1300 mW Linewidth: 100 kHz AOM Wavemeter RF Synthesizer PMT Photon Counting unit Frequency Counter (f rep, f CEO ) DG AOM: Acousto-optic modulator PBS : polarizing beam splitter PCF : photonic crystal fiber DG : diffractive grating APD: avalanche photo diode PMT: Photomultiplier tube APD Iodine Saturated Absorption Spectroscopy GPS clock Experimental Setup Frequency Counter (beat frequency) 6

0 Spectral Intensity 160 Frequency[MHz] (RBW ： 300 kHz) f rep = MHz Frequency Measurement with a Comb Spectral Intensity Frequency[MHz] f dye f rep fnfn f n+1 f dye - f n f n-1 - f dye Frequency f n+2 Laser spectra in optical region Beat spectrum in RF region 7

Doppler-free Two Photon Absorption Spectroscopy Ti:S Comb Power : 400 mW nm Ti:S Comb Power : 400 mW nm PC λ/4 λ/2 PBS PCF Single mode cw dye Laser Power : 1300 mW Linewidth: 120 kHz Single mode cw dye Laser Power : 1300 mW Linewidth: 120 kHz AOM Wavemeter RF Synthesizer PMT Photon Counting unit Frequency Counter (f rep, f CEO ) DG AOM: Acousto-optic modulator PBS : polarizing beam splitter PCF : photonic crystal fiber DG : diffractive grating APD: avalanche photo diode PMT: Photomultiplier tube APD Iodine Saturated Absorption Spectroscopy GPS clock Experimental Setup Frequency Counter (beat frequency) 8

Frequency Measurement with the Comb and an AOM f dye f dye +f AOM AOM shift f AOM f rep f rep /4 ～ 40 MHz 0160 [MHz] f rep fnfn f n+1 f n+2 f dye = f n - f AOM + f beat Absolute frequency of dye laser Uncertainty ～ ～ 5 kHz 9

[MHz] AOM frequency shift Beat frequency I 2 spectrum Wavenumber [cm -1 ] [GHz] [arb.unit] Saturated Absorption Spectrum of I 2 ～ P(62)17-1 Scan rate : 0.8 MHz/s, Gate time of the counter : 0.1 s Wave length : 576 nm Measurement time : 22 minutes a1a1 10

[MHz] HWHM =1.5 MHz Absolute Frequency [MHz] Previous measurement [1] (12) CIPM recommended value [2] (2) [1] A. Nishiyama, et. al., J. Opt. Soc. Am. B 30, 2107 (2013). [2] T. J. Quinn, Metrologia 40, 103–133 (2003). Saturated Absorption Spectrum of I 2 ～ a 1 component 11

Doppler-free Two Photon Absorption Spectroscopy f dye u f dye (1-u/c)f dye (1+u/c) Dye Laser Linewidth:120 kHz Dye Laser Linewidth:120 kHz PMT PD λ/4 λ/2 20 Pa 58 W fluorescence 596 nm 298 nm S1S1 S0S0 － +－ + Hänsch-Couilland scheme 12

x z Two photon transition of naphthalene D 2h S11B1uS11B1u S01Ag S01Ag v 4 (C-C stretch)=1 : b 1u ～ cm -1 ～ cm -1 ～ cm -1 S21B3uS21B3u 1560 cm -1 Vibronic interaction →Allowed transition 13

Naphthalene Spectrum Wavenumber [cm -1 ] S 1 1 B 1u (v 4 =1 : b 1u ) ← S 0 1 A g (v=0) transition Band origin Scan rate : 2 MHz/s, Gate time of the counter : 0.2 s Photon count [/0.2 s] Frequency shift [GHz] 14

Linewidth of Naphthalene spectra ＜ Q(K) Q(J) transitions （ ΔJ = 0, ΔK = 0 ） ＞ [cm -1 ] J = 14 Kc = J Ka = 0,1 HWHM = 1.24 MHz Natural width [1] = 0.91 MHz [1] U. Boesl et. al. Chem. Phys. Lett, 42, 16 (1976). S 1 1 B 1u (v 4 =1 : b 1u ) a b c [count] Resolution = 0.4 MHz [cm -1 ] J = Kc = J Ka = 0,1 0 Photon count [/0.2 s] 15

Linewidth of Naphthalene spectra ～ pressure broadening Scan rate : 1 MHz/s, Gate time of the counter : 0.1 s [Pa] [MHz] Natural width [1] 0.91 MHz [cm -1 ] [count] 16

Calculation of Rotational Constants S 1 B 1u (v 4 =1 ) This work [cm -1 ] S 0 A g (v=0 ) ※ [cm -1 ] A B C A-(B+C)/ (92) (B+C)/ (17) (B-C)/ (27) Dk× (77)1.87 Djk× (30)0.118 Dj× (48)0.580 Hk× (18) Hkj× (13) Hjk× (35) Hj× (31) dk× (44)0.137 dj× (31)0.159 hk× (26) hjk× (37) hj× (17) ※ K. Yoshida, et al., J. Chem. Phys. 130, (2009). Least-squares fittings of the theoretical energies (840 lines) Ka + Kc = J + 1 [MHz] J Ka = 0 Ka = 1 Ka = 2 Ka = 3 Ka = 4 Ka + Kc = J Obs.- Cal

● We developed a precise frequency measurement system for frequency scanning cw lasers with an optical frequency comb. Precision = 5 kHz ● To realize high resolution narrower than the natural width, we adopted the Doppler-free two photon absorption technique. Resolution = laser linewidth ～ 100 kHz ● We applied the developed system to high resolution spectroscopy of naphthalene. ・ Theoretical analysis of obtained spectra in high rotational levels ・ Dynamics in excited states Summary 18

Range ： ~ cm -1 (1.5 GHz) Time ： 25 s f rep = ± Hz f CEO = ± MHz BPF1 BPF Sat. Abs. [arb. unit] Wavenumber [cm -1 ] mode number n : mode frequency f n : [MHz] f rep f BPF1 f BPF2 System I ～ I 2 Saturated Absorption Spectrum ～ P(62)17-1

Ti:S Comb Power : 400 mW nm Ti:S Comb Power : 400 mW nm Dye Laser Power : 1300 mW Linewidth: 120 kHz Dye Laser Power : 1300 mW Linewidth: 120 kHz Wavemeter Frequency Counter (f rep, f CEO ) DG BPF : Band-Pass Filter ED : Envelope detector PCF : photonic crystal fiber APD I 2 Saturated Absorption Spectroscopy GPS clock PBS AOM APD Lock-in Amplifier Lock-in Amplifier PCF LPF ED BPF1BPF2 System I ～ Experimental Setup