Transcript שקופית 1 - huji.ac.il

Jonathan Gorodetsky, Leonid Belau and Yehuda Haas
Hebrew University of Jerusalem, The Farkas Center for Light-Induced Processes
Goals:
0
3E-5
23
195
2E-4
Using a TOF-MS constructed to detect neutral or positive species we studied the laser
ablation products of alkali azide salts. By cooling the ablated material we hoped to
detect stable HEDM (High Energy Density Materials).
23
194
Ablation Products from sodium azide:
neutral and positive species
Comparison between NaN3 Ablation products
skimmer
Ablation laser
140
160
Na(NaN 3)
130
131
118
127
128
103
104
105
Na(H 2O)4
95
96
81
78
86
77
88
72
64
112
Na(NaCN)
Na(NaOH)
59
60
51
46
64
105
102
118
119
120
115
112
113
106
107
109
110
101
100
92
89
84
85
86
87
82
83
81
103
88
72
62
104
39
63
Na(H 2O)2
46
41
55
60
65
70
75
80
85
90
95
100
105
110
115
120
Mass (amu)
37
200
250
300
350
400
450
500
550
600
650
700
50
313
315
Na(Na Cl)2(Na Cl)2
35
Na(Na Cl)4
Na(Na Cl)3
Na(Na Cl)3
197
1351
199
37
35
1312
1252
35
255
257 Na(Na 35Cl)3(Na37Cl)
259
35
37
1478
Na(Na Cl)2
37
Na(Na Cl)2
100
Ion mass (AMU)
Mass (amu)
Fig. 4: Identification of nitrogen in the ablated material was done by
substituting sodium azide with sodium azide labeled by a single 15N
isotope. Only the peaks at 72 amu and 88 amu were shifted, confirming that
only these compounds contain nitrogen. The relative intensities of the split
peaks allow us to speculate on the nitrogen content of those peaks.
141139
1152
46
0.0
150
20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120
1023
0.4
27
0.6
0.2
0.0
63 Na(NaOH)
72 Na(NaCN)
0.8
0.2
0.0
83
1.0
NaN3 +CHCl3
Na(Na Cl)
23
Relative intensity (a.u)
818
834
626 Cs(CsN 3)(CsCN) 2
643 Cs(CsN 3)2(CsCN)
658 Cs(CsN 3)3
Cs(CsN 3)2
Cs(CsN 3)(CsCN)
Cs(CsN 3)(CsOH)
442
451458
467
483
Cs(CsCN)(CsOH)
Cs(CsCN) 2
292 Cs(CsCN)
308 Cs(CsN 2)
325 (CsN 3)(CsOH)
123
103
88
89
283
Cs(CsOH)
0.6
0.2
81 Na(Na 35Cl)
133 Cs
0.8
266 Cs2
14
Na N N2
73
65
1.4
1.0
72
46
49
53
Sodium azide ablation products seeded
with chloroform
1.2
151 Cs(H 2O)
169 Cs(H 2O)2
187 Cs(H 2O)3
Ion intensity (A.U)
88 Na(NaN 3)
77
1.2
0.4
0.6
Fig. 3: To support peak assignment the molecular jet was seeded with D2O.
Top: sodium azide seeded with H2O. Bottom: sodium azide seeded with
D2O. All the peaks associated with species containing hydrogen were
shifted due to the substitution of hydrogen with deuterium. Compounds that
do not contain hydrogen such as Na(NaCN) and Na(NaN3) are not shifted.
signal
1.0
39
41
50
198
1.4
Na N3
15
63
1.2
27
200
Cation ablation products of Cesium Azide
46
0.1
1.4
180
Fig. 2: Mass spectra of the ablation products from sodium azide. Top:
neutral ablation products. Bottom: positive ablation products. Both spectra
display similar peaks whose assignments can be found in table 1 and on the
graphs. The most prominent are Na with a mass of 23 amu, Na(NaN3) with
a mass of 88 amu, Na(NaOH) with a mass of 63 amu and oddly Na(NaCN)
with a mass of 72 amu.
86
72 Na(NaCN)
63
51
41
0.3
38
63
120
23
0.5
Relative intensity (a.u)
00 0
00
0 0 00
100
881136
2E-4
0.7
12
80
14
0.9
Relative intensity (a.u)
60
laser
1.1
0.4
40
Mass (amu)
Comparison of neutral ablation products from sodium azide
and labeled sodium azide
0.8
0.0
20
Fig. 1: Experimental setup: A high power laser ablates the sample. The
reactive products are cooled by an expanding super-sonic jet. Ablated
material entrained in the jet is ionized by a second laser, accelerated, and
continues in uniform motion in the drift tube. Finally the particles are
detected by a standard Daly detector. Alternatively positively charged
ablated material can be detected by pulsing the voltage on the plates.
1.3
0.4
0.2
0.0
Ionization
D2O seeded
80
0.2
H2O seeded
0.6
69
0.4
0.2
61
gas valve
46 Na2
0.6
23 Na
27 Al
0.8
Cations
195 Pt
1.0
Acceleration
plates
88 Na(NaN 3)
0
1.2
00
0 00
1.4
0
reactor
0.1
1.6
88 Na(NaN 3)
sample
holder
0.3
72
Signal path
0.5
63 Na(NaOH)
Drift tube
trigger
0.7
46 Na2
NaN3
0.9
41 Na(H 2O)
Relative intensity (a.u)
Particle path
77 Na(H 2O)3
1.1
Na(NaCN)
1.3
0.4
Neutals
72 Na(NaCN)
1.5
196
63 Na(NaOH)
Daly detector
23 Na
7E-4
seeded with D2O and H2O
Na(H 2O)3
Oscilloscope &
Data acquisition
150
200
250
300
Mass (amu)
Fig. 5: Replacing sodium azide with cesium azide provides much in-sight
into the chemistry of the ablation products. No peaks were shared by
sodium azide and cesium azide (the first peak in cesium azide appears at
133 amu), indicating that all the peaks must contain an alkali atom. Not
surprisingly, the clusters detected in cesium azide can be correlated to
clusters detected in sodium azide.
Fig. 6: To determine reaction mechanisms, chloroform was seeded in the
helium jet. Strong peaks attributed to Na(NaCl)n appeared up to high
masses (n>7). All the characteristic sodium azide peaks also remained. This
shows that reactions do take place in the gas phase. The sodium chloride
peaks split statistically according to the natural abundance of chloride
isotopes, indicating a purely statistical mechanism of formation.
Discussion
• Most detected products can be assigned to clusters such as M(MX)n were M is an
1
halogen .
alkali metal and X is halogen or pseudo
The most probable assignments are
given in table 1.
• The presence of (MCN)n and (MOH)n clusters indicate that reactions do take place
after ablation.
• Carbon and water in the ablated species may be the result of occluded H2O and
CO2 in the crystal2.
• Seeding by CHCl3 demonstrates how clusters are formed. The intensity distribution
of the clusters indicate a statistical mechanism of formation, as can be seen by table
2.
• Comparison of cesium azide and sodium azide ablation products show that all the
products contain an alkali metal.
• Isotope substitution experiments imply that no nitrogen rich material is formed
Conclusion
HEDM were not detected by ablating alkali azide metals. Our experiments show that reactions do take place after ablation,
however impurities have a significant effect on the end products. It is our intention to try and nullify the effect of such
impurities in further experiments
1. Y. J. Twu, C. W. S. Conover, Y. A. Yang and L. A. Bloomfield, Phys. Rev. B, 42. 5306 (1990).
Acknowledgments
2. R. F. Walker, N. Gane and F. P. Bowden, Proc. Roy. Soc. A 294, 417 (1966).
This project was sponsored by the Israeli Ministry of Defense