Time-resolved Fourier transform infrared emission spectra of HNC/HCN

K. Kawaguchi & A. FujimotoOkayama University

Introduction　 HNC : metastale isomer of HCN 0.62 eV higher energy isomerization reaction

　　 In low-temperature interstellar clouds [HNC] 〜 [HCN] (T. Hirota, ApJ, 1998) 　　 Branching ratio in recombination reaction HCNH+ + e → 　 HCN, HNC, CN

[HCN]/[HNC] 〜 3 (T, Amano et al. 2004) Time-resolved Infrared emission spectra

Data Sampling in a FT Spectrometer

He-Ne Laser

Samplingfor IR data

= 15803 cm-1

< 8000 cm-1

SpectrumFourier Transform

Interferogram

Path difference

- 200 - 100 0 100 200 300 400

Time (sec)

Timing diagram for TRFTSScan signal

He-Ne laser

Pulse event

Sampling in usual TRFTS

Sampling in our system

T Many scansare required

One scan

Block diagram of TRFTS system

ADC 432216 bit

≲ 2 MHz

New methodSX is replaced by FPGA(Field Programmable Gate Array)

Pulse discharge and time resolved spectra

Scan

He-Ne

DischargeTrigger

AD Trigger

100sec

(64)

64 interferograms

Fourier Transform

Max. 64 Spectra at preset time intervals

Bruker

Bruker

SX

SX

PC, C++

PC, C++

Performance of time-resolved FT

1. wavenumber resolution : 0.008 cm-1

2. time resolution : 0.5 sec (FPGA)3. pulse event frequency : < 40 kHz4. number of time-resolved spectra resolution : up to 0.04 cm-1 : 64 0.016 cm-1 : 32 0.008 cm-1 : 16

Emission

CH4（ 15mT）C2H3CN（ 10mT）He （ 1.35 T ）　 N2 （ 120 mT ）

He（ 6 T）　H2（ 60 mT）

anodeHe

watercathode

Expanded polystyrene

Pump

Liq.N2

Dry ice（ in ethanol）

Window

discharge

Emission cell

Emission spectra of HCN, HNC ( at 77K) (32s)

HCN

HNC

P(9)

P(17)

P(3)R(8)R(2

)R(15)

P(9)

P(3)P(14)

R(9)

R(16)

R(3)

1(100-000)

(101-001)

(200-100)

1(100-000)

(101-001)

(200-100)

3050 3100 3150 3200 3250 3300 3350 34000

10 126 sec

wavenumber (cm- 1)

0

10 102 sec

0

10

0

10

78 sec

54 sec

0

10

0

10

30 sec

6 sec

HCN emission

200 KCH4 + N2

60 points3 sec step

0-180 sec covered

(300-200) (200-100) (100-000)

0 20 40 60 80 100 120 140 160 180 200

123456789

101112131415161718

HCN (300) - (200)

HCN (200)- (100)

HCN (100)- (000)em

ission

inte

nsity

time (sec)

Discharge 0-20 sec 200 K CH4 + N2

Vibrationalrelaxation

5 times shorterthan CO

Time variation of emission spectra (at 77 K)HCN

N2 HNC

2s

12s 22s

32s

42s

52s

Time variation of emission spectra (at 200 K)

2s

12s

22s

32s

42s

52s

HCN

HNC

（ emission intensity of hot band : x 3 ）

Time profile of emission intensities of HCN, HNC

liq. N2

Temp.

Dry iceTemp.

0 10 20 30 40 50 603000

3500

4000

4500

5000

5500

6000

dischargeon

HNC (200)- (100) (x 3)

HCN (200)- (100)em

ission

inte

nsity

time (sec)

Decay from (200) states of HNC and HCN

77 K

3316 3317 3318 3319 3320 3321

0

5000

10000

HCN Liq. N2 Temp.

30 spectra are shown by different colors

N2 electronic

3316 3317 3318 3319 3320 3321

0

5000

10000

3316 3317 3318 3319 3320 3321

0

2000

4000

HCN emission : Comparison in two conditions

77 KCH4 + N2

200 KC2H3CN+ H2

(100-000)(101-001)

(110-010)

3622 3623 3624 3625 3626- 1000

0

1000

2000

3000

4000

5000

6000

7000

3622 3623 3624 3625 3626

0

400

800

77 KCH4 + N2

200 KC2H3CN+ H2

HNC emission (100-000) P(9)(101-001)P(6)

N2

Decay from (101) is fastercompared with above

Temperature and/or chemical

Results (1)Tvib, Trot ( 　 2s after turning off the discharge)　　　● liq. N2 temp.

Tvib ・・・ HCN 3350 (± 250) K HNC 2200 K Trot ・・・ HCN 263 (± 2) K HNC 340 (± 40)

K　　　● dry ice temp. Tvib ・・・ HCN 2750 (± 250) K HNC 2400 K Trot ・・・ HCN 379 (± 6) K HNC 480 (± 7) K

Emission : increased after discharge-offIf the increment is due to reactions, abundance ratio　　　● liq. N2 temp.　　　　 [HCN] : [HNC] = 40 : 4.3　　　● dry ice temp.　　　　 [HCN] : [HNC] = 10 : 1

Results (2)1. decay from HNC (200) is fast (10 sec) compared with that of HCN (200) HNC – HCN conversion?2. bending excited states : up to v2=2 (120)

production vibrational relaxation 3. decay from (101) of HNC is fast in (C2H3CN + H2) discharge at 200 K

vibrational relaxation

HNC-HCNconversion

HNC

HCN

AcknowledgmentsK. Manabe Ibaraki UniversityT. Amano (Ibaraki University)