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

S1

S0

S2

T1

T2

Vibrationalstructure

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

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

fn = nfrep + fCEO

（ n : mode number ）　

fCEO (Carrier-Envelope Offset)

frep (Repetition rate ) fn

→ Precise optical frequency ruler

Sp

ect

ral

Inte

nsi

ty

Optical Frequency

Introduction ～ Optical Frequency Comb

GPS clock 　　～ 10-11

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-freeTwo Photon Absorption

Spectroscopy

Ti:S Comb Power : 400 mW 　 600-1200 nm

PC

λ/4

λ/2PBS

PCF

Single mode cwdye Laser

Power : 1300 mWLinewidth: 100 kHz

AOM

Wavemeter

RFSynthesizerPMT

Photon Counting unit

Frequency Counter(frep, fCEO)

DG

AOM: Acousto-optic modulatorPBS : polarizing beam splitterPCF : photonic crystal fiberDG : diffractive gratingAPD: avalanche photo diodePMT: Photomultiplier tube

APD

Iodine Saturated Absorption Spectroscopy

GPS clock

Experimental Setup

Frequency Counter

(beat frequency)

6

0Sp

ectr

al

Inte

nsi

ty

160Frequency[MHz]

(RBW ： 300 kHz)

frep= 162.5 MHz

Frequency Measurement with a Comb

0 160 0 160 0 160Sp

ectr

al

Inte

nsi

ty

Frequency[MHz]

fdye

frep

fn fn+1

fdye- fnfn-1- fdye

Frequency

fn+2

Laser spectra in optical region

Beat spectrum in RF region

7

Doppler-freeTwo Photon Absorption

Spectroscopy

Ti:S Comb Power : 400 mW 　 600-1200 nm

PC

λ/4

λ/2PBS

PCF

Single mode cwdye Laser

Power : 1300 mWLinewidth: 120 kHz

AOM

Wavemeter

RFSynthesizerPMT

Photon Counting unit

Frequency Counter(frep, fCEO)

DG

AOM: Acousto-optic modulatorPBS : polarizing beam splitterPCF : photonic crystal fiberDG : diffractive gratingAPD: avalanche photo diodePMT: Photomultiplier tube

APD

Iodine Saturated Absorption Spectroscopy

GPS clock

Experimental Setup

Frequency Counter

(beat frequency)

8

Frequency Measurement with the Comb and an AOM

fdye fdye+fAOMAOM shift fAOM

frep

frep/4 ～ 40 MHz

0 160[MHz]

frepfrepfrep

fn fn+1 fn+2

fdye = fn - fAOM + fbeat

Absolute frequency of dye laser

Uncertainty ～ 10-11

～ 5 kHz 9

40.8

41.2

[MHz]

AOMfrequency shift

Beatfrequency

1.16

1.00

I2 spectrum

17352.23 .24 .25 .26Wavenumber [cm-1]

[GHz]

[arb.unit]

Saturated Absorption Spectrum of I2 ～ P(62)17-1

Scan rate : 0.8 MHz/s, 　 Gate time of the counter : 0.1 sWave length : 576 nm Measurement time : 22 minutes

a1

10

520 206 808.470520 206 790 520 206 825 [MHz]

HWHM =1.5 MHz

Absolute Frequency [MHz]

Previous measurement[1]

520 206 808.45 (12)

CIPM recommended value[2]

520 206 808.4 (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 I2 ～ a1 component

11

Doppler-free Two Photon Absorption Spectroscopy

fdye

u

fdye(1-u/c) fdye(1+u/c)

Dye LaserLinewidth:120 kHz

PMT

PDλ/4

λ/2

20 Pa

58 W

fluorescence

596 nm

596 nm

298 nm

S1

S0 － + Hänsch-Couilland scheme

12

x

z

Two photon transition of naphthalene

D2h

S11B1u

S01Ag

v4 (C-C stretch)=1 : b1u ～ 33578 cm-1

～ 32018 cm-1

～ 35804 cm-1S21B3u

1560 cm-1

Vibronic interaction→Allowed transition

<Two photon transitions>

13

Naphthalene Spectrum

Wavenumber [cm-1]

33577.0 33577.5 33578.0

S11B1u (v4=1 : b1u) ← S0

1Ag (v=0) transition

Band origin

Scan rate : 2 MHz/s, 　 Gate time of the counter : 0.2 s

Ph

oto

n c

ou

nt

1.16

1.00

[/0.2 s]

Fre

qu

en

cy

sh

ift

[GHz]

14

Linewidth of Naphthalene spectra

＜ Q(K)Q(J) transitions （ ΔJ = 0, ΔK = 0 ）＞

5000

033577.9150

.9160

.9155

[cm-

1]

J = 14Kc = JKa = 0,1 HWHM

= 1.24 MHz Natural width[1] = 0.91 MHz[1] U. Boesl et. al. Chem. Phys. Lett, 42, 16 (1976).

S11B1u (v4=1 : b1u)

a

b

c

[count]

Resolution = 0.4 MHz

33577.85 [cm-1].90 .95

1011121314151617J = 18

5000

Kc = JKa = 0,1

0

Photon count[/0.2 s]

15

Linewidth of Naphthalene spectra ～ pressure broadeningScan rate : 1 MHz/s, 　 Gate time of the counter : 0.1 s

10 20 30 400 [Pa]

1.0

1.2

1.4

0.8

1.6

1.8

[MHz]

Natural width[1]

0.91 MHz

<HWHM of Q(0,1)Q(14) line>

33577.9150

.9160.9155 [cm-1]

[count]

16

Calculation of Rotational Constants

S1B1u(v4=1 )This work [cm-

1]

S0Ag(v=0 )

※ [cm-1]A 0.10137658 0.104052B 0.04043669 0.041127C 0.02893318 0.029484

A-(B+C)/2

0.06669165(92)

(B+C)/2 0.03468493(17)

(B-C)/2 0.00575176(27)

Dk×108 -7.41(77) 1.87Djk×108 2.95(30) 0.118Dj×109 0.95(48) 0.580Hk×1010 -4.32(18)Hkj×1010 3.34(13)Hjk×1011 -6.63(35)Hj×1012 1.70(31)dk×108 8.06(44) 0.137dj×109 3.03(31) 0.159hk×1010 -4.08(26)hjk×1013 -0.13(37)hj×1014 0.27(17)

※K. Yoshida, et al., J. Chem. Phys. 130, 194304 (2009).

Least-squares fittingsof the theoretical energies

(840 lines)

Ka + Kc = J + 1

[MHz]

J 0 30 10 20

Ka = 0

Ka = 1

Ka = 2

Ka = 3

Ka = 4

Ka + Kc = J Obs.- Cal.

0

200

-200

0

200

-200

0

200

-200

0

200

-200

0

200

-200

17

● 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

<Future Plan>

18

Range ： 17352.21 ~ 17352.26 cm-1 (1.5 GHz)Time ： 25 sfrep = 162 525 957.486 ± 0.005 HzfCEO = 54.173 ± 0.004 MHz

BPF1BPF2

17352.22 .23 .24 .25 .26 .27

Sat.

Ab

s.

[arb

. u

nit

]

Wavenumber [cm-1]

mode number n : 3200761mode frequency fn : 520 206 800.38

[MHz]

frep fBPF1

fBPF2

System I ～ I2 Saturated Absorption Spectrum ～ P(62)17-1

Ti:S Comb Power : 400 mW 　 600-1200 nm

Dye LaserPower : 1300 mWLinewidth: 120 kHz

Wavemeter

Frequency Counter(frep, fCEO)

DG

BPF : Band-Pass FilterED : Envelope detectorPCF : photonic crystal fiber

APD

I2 Saturated Absorption Spectroscopy

GPS clock

PBS

AOM

I2 Cell

APD

Lock-inAmplifier

PCF

LPFED

BPF1

BPF2

System I ～ Experimental Setup