Using a time-of-flight mass spectrometer, we have measured the threshold for producing ions from various aerosol particles in a vacuum with laser radiation at 248 nm, 308 nm, and 10.6 μm. In addition, a limited amount of similar data were taken at 193 and 337 nm. At 10.6 μm, two thresholds were observed: one near 3 GW/cm2, which corresponds to partial ionization, and another at 6 GW/cm2, which we attribute to plasma formation. At 308 nm, the threshold for ion production is on the order of 200 MW/cm2. Shorter wavelengths require even less energy, with < 100 MW/cm2 yielding normal molecular-mass spectra and approximately 500 MW/cm2 fragmenting the sample to atomic ions.
Ronald G. Pinnick, Petr Chýlek, M. Jarzembski, E. Creegan, V. Srivastava, Gilbert Fernandez, J. D. Pendleton, and A. Biswas Appl. Opt. 27(5) 987-996 (1988)
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Thresholds measured for various particles with 10.6 μm CO2 laser radiation. The less than sign (<) preceding a number indicates that the measured values must be considered only as upper limits. Relative uncertainties for these thresholds are ±25%; absolute uncertainties are ±65%. Relative uncertainties for particle diameters are ±10%, and absolute uncertainties are ±30%.
In joules per square centimeter and gigawatts per square centimeter.
Table 2
Ion-Production Thresholds for Various Particles with 308-nm Excimer-Laser Radiationa
Substance
Particle Diameter (μm)
Ion-Formation Threshold
J/cm2
MW/cm2
Oleic acid
4.3
4.2
130
Oleic acid
8.5
17.2
520
Oleic acid
14.6
25.7
770
Oleic acid + NaI (10%)
8.6
5.3
160
Dioctyl sebacate
4.2
2.3
70
Dioctyl sebacate
8.4
14.7
440
Dioctyl sebacate
17.0
31.5
950
Dioctyl sebacate + NaI (5%)
8.5
7.1
210
Dioctyl sebacate + NaI (1%)
17.0
5.4
160
Methyl salicylate
12.5
19.7
590
Vitamin E
4.2
0.15
4.5
Vitamin E
8.4
0.42
13
Vitamin E
16.8
0.63
19
Vitamin E + NaI (25%)
4.3
0.12
3.6
Vitamin E + NaI (5%)
8.4
0.60
18
NaI
2.8
0.86
26
NaI
5.6
0.74
22
NaI
11.7
0.92
28
Ammonium sulfate
3.5
12.7
380
Ammonium sulfate
7.1
16.1
480
Ammonium sulfate
11.5
16.2
490
Relative uncertainties for particle diameters are ±10%; absolute uncertainties are ±30%. Relative uncertainties for thresholds are ±35%; absolute uncertainties are ±55%.
Table 3
Ion-Production Thresholds for Various Particles with 2.5-ns Pulses of 248-nm Laser Radiationa
Substance
Particle Diameter (μm)
Ion-Formation Threshold
J/cm2
MW/cm2
Oleic acid
3.4
0.28
112
Oleic acid
6.7
0.42
168
Oleic acid
13.4
1.09
436
Vitamin E
3.3
0.10
40
Vitamin E
6.6
0.10
40
NaI
2.2
0.14
56
NaI
4.4
0.13
52
KI
2.3
0.05
20
KI
4.6
0.14
56
KI
9.4
0.21
84
Ammonium sulfate
5.6
0.17
68
Relative uncertainties for particle diameters are ±10%; absolute uncertainties are ±30%. Relative uncertainties for thresholds are ±35%; absolute uncertainties are ±55%.
Table 4
Estimated Thresholds for Ion Formation for Five Laser Wavelengthsa
Laser Wavelength (nm)
Ion Formation Threshold
J/cm2
MW/cm2
197
0.3
25
248 in 10-ns pulses
0.5
50
248 in 2.5-ns pulses
0.20
80
308
17
500
337
>0.2, <6
>20, <600
10,600
370
2800
Uncertainties are on the order of a factor of 3 for data at 197 nm and for 10-ns pulses at 248 nm. Limited data at 337 nm allows only for a lower and an upper limit on the threshold. Typical particles used were 7-10-μm-diameter oleic acid and ammonium sulfate aerosols.
Tables (4)
Table 1
Thresholds for Visible Emission, Ion Production, and Plasma Formationa
Thresholds measured for various particles with 10.6 μm CO2 laser radiation. The less than sign (<) preceding a number indicates that the measured values must be considered only as upper limits. Relative uncertainties for these thresholds are ±25%; absolute uncertainties are ±65%. Relative uncertainties for particle diameters are ±10%, and absolute uncertainties are ±30%.
In joules per square centimeter and gigawatts per square centimeter.
Table 2
Ion-Production Thresholds for Various Particles with 308-nm Excimer-Laser Radiationa
Substance
Particle Diameter (μm)
Ion-Formation Threshold
J/cm2
MW/cm2
Oleic acid
4.3
4.2
130
Oleic acid
8.5
17.2
520
Oleic acid
14.6
25.7
770
Oleic acid + NaI (10%)
8.6
5.3
160
Dioctyl sebacate
4.2
2.3
70
Dioctyl sebacate
8.4
14.7
440
Dioctyl sebacate
17.0
31.5
950
Dioctyl sebacate + NaI (5%)
8.5
7.1
210
Dioctyl sebacate + NaI (1%)
17.0
5.4
160
Methyl salicylate
12.5
19.7
590
Vitamin E
4.2
0.15
4.5
Vitamin E
8.4
0.42
13
Vitamin E
16.8
0.63
19
Vitamin E + NaI (25%)
4.3
0.12
3.6
Vitamin E + NaI (5%)
8.4
0.60
18
NaI
2.8
0.86
26
NaI
5.6
0.74
22
NaI
11.7
0.92
28
Ammonium sulfate
3.5
12.7
380
Ammonium sulfate
7.1
16.1
480
Ammonium sulfate
11.5
16.2
490
Relative uncertainties for particle diameters are ±10%; absolute uncertainties are ±30%. Relative uncertainties for thresholds are ±35%; absolute uncertainties are ±55%.
Table 3
Ion-Production Thresholds for Various Particles with 2.5-ns Pulses of 248-nm Laser Radiationa
Substance
Particle Diameter (μm)
Ion-Formation Threshold
J/cm2
MW/cm2
Oleic acid
3.4
0.28
112
Oleic acid
6.7
0.42
168
Oleic acid
13.4
1.09
436
Vitamin E
3.3
0.10
40
Vitamin E
6.6
0.10
40
NaI
2.2
0.14
56
NaI
4.4
0.13
52
KI
2.3
0.05
20
KI
4.6
0.14
56
KI
9.4
0.21
84
Ammonium sulfate
5.6
0.17
68
Relative uncertainties for particle diameters are ±10%; absolute uncertainties are ±30%. Relative uncertainties for thresholds are ±35%; absolute uncertainties are ±55%.
Table 4
Estimated Thresholds for Ion Formation for Five Laser Wavelengthsa
Laser Wavelength (nm)
Ion Formation Threshold
J/cm2
MW/cm2
197
0.3
25
248 in 10-ns pulses
0.5
50
248 in 2.5-ns pulses
0.20
80
308
17
500
337
>0.2, <6
>20, <600
10,600
370
2800
Uncertainties are on the order of a factor of 3 for data at 197 nm and for 10-ns pulses at 248 nm. Limited data at 337 nm allows only for a lower and an upper limit on the threshold. Typical particles used were 7-10-μm-diameter oleic acid and ammonium sulfate aerosols.
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