1. Time-Resolved IR and Mass Spectroscopy of Laser-Ablated Magnesium
Yuki Miyamoto, Chisato Masaki, Naomi Ikeda, Jian Tang
and Kentarou Kawaguchi
Okayama University
67th International Conference on Molecular Spectroscopy, The Ohio State University, USA, June 18 – 22, 2012

2. Motivation Mg The most abundant metal in the Universe Laser Ablation
Solar-system abundances of the elements
The most abundant metal in the Universe
the 9th in mass fraction and the 8th in atom fraction
Geochimica et Cosmochimica Acta 46, 2363 (1982)
Group 2 Period 3 Mg
~Si, Fe 12 Mg
4th abundant in the Earth
8th abundant  in the Earth's crust
5th abundant in the Ocean
11th abundant in Human body
Magnesium
24.31
Laser Ablation
Widely used technique in various fields
Easy (no sample-size requirement, no complicated sample preparation)
Applicable to any material
There are few IR spectroscopic studies on Laser-ablated metals, although there are many works in optical region.
This study
Time-resolved IR spectroscopy of Laser-ablated Mg

3. Three Experiments Time-Resolved FTIR spectroscopy
Mass measurement using a Q-Mass detector
Time-Resolved cw IR laser spectroscopy

4. Time-Resolved FTIR spectroscopy
Temporal behavior of IR emission/absorption of laser-ablated Mg
cm-1
time resolution ~ μs
freq. resolution ~ 0.01 cm-1
Ablation Laser
Nd:YLF SHG
526.5 nm
1~3 W
1 ~ 3 KHz

5. FTIR Spectra 12 emission lines
5p-5s
5g-4f
5d-5p
4p-3d
5f-4d
5s-4p
4d-4p
~ 50,000 cm-1
*Mg ground state (2p)6(3s)2
Emission of highly-excited Mg were observed.

6. Time-Resolved Spectra
Two distinct components with different Doppler width were observed.
Temporal change of 5g-4f transition
Triplet
Singlet
Broad component ~ 0.1 cm-1
Emission  Absorption
Narrow component ~ 0.02 cm-1
Emission only
Why two components?
Origins of atoms
I. Direct desorption
II. Via some reactions ?
dissociative recombination of molecular ions
Mg2+ + e-  2Mg
recombination
Mg+ + e-  Mg
Further information  mass measurement / laser spectroscopy

7. Mass spectroscopy with Q-Mass
What species are produced by ablation ?
Mass range 1-200
Mass resolution < 1
Ablation Laser
Nd:YLF SHG
526.5 nm
1~3 W
1 ~ 3 KHz
Turning On/ Off of ionization filament
neutral / ion
Effect of residual or leaked air
 adding O2 and N2.

8. Mass Spectra FIL off FIL on FIL off Mg+ and Mg2+ were observed. Mg+ Mg
0.5mJ/pulse
Mg+ and Mg2+ were observed.

9. Oxygen/Nitrogen
~effect of residual or leaked air
Adding Oxygen
FIL off
FIL off
Mg2+
MgO+
O2 ~ 10-4 Torr
O2 ~ 10-5 Torr
O2 ~ 10-5 Torr
O2 ~ 10-4 Torr
*Nitrogen did not effect spectra.
MgO+ was produced by adding O2 while Mg2+ was destructed.

10. Time-Resolved IR Laser Spectroscopy
High sensitivity, high resolution
time resolution ~ 0.1 μs
freq. resolution ~ cm-1
IR Laser
Fiber laser pumped
PPLN OPO
3~4.5 μm
~200 mW
Ablation Laser
Nd:YLF SHG
526.5 nm
1~3 W
1 ~ 3 KHz

11. 2D Spectra
5g-4f (triplet)
emission
absorption
Stimulated emission
5g-4f (triplet) transition consists of broad absorption (0.2 cm-1) and narrow stimulated emission (~0.02cm-1).
~0.02 cm-1
absorption
~0.2 cm-1

12. Oxygen
Adding Oxygen
Oxygen
No significant change by adding oxygen

13. Broad and Narrow Components
Broad-width component
width 0.1~0.2 cm-1 (10,000~20,000 K)
FTIR
Spontaneous emission
 Absorption
Temperature estimated from Doppler width
is comparable to energy of states(~ 70,000 K)
Laser
Only Absorption
Thermal distributed
direct desorption
Narrow-width component
width 0.02 cm-1
Stimulated emission
 Non-thermal distribution
FTIR
Spontaneous emission
Laser
Stimulated emission
No dependence on O2
Due to recombination
×Dissociative recombination of Mg2+ or MgO+  should depend on amount of O2
✔Recombination Mg+ + e-  Mg*
Only slow Mg+ can recombine with electrons.  narrow width emission

14. Summary
Time-resolved IR spectroscopy and mass spectroscopy were performed on laser-ablated Mg.
Mass spectra shows that major products are Mg, Mg+ and Mg2+. By adding O2, MgO+ is produced and Mg2+ is destructed.
Observed IR emission / absorption of Mg were consist of two components with different Doppler width.
Broad-width component = thermal, Narrow-width component = Mg++e- recombination.