J . Phys. Chem. 1993,97, 10295-10300
10295
Reactions of Pulsed-Laser Ablated A1 Atoms with H2. Infrared Spectra of AlH, AlH2, AlHs, and A12H2 Species George V. Chertihin and Lester Andrews' Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 Received: April 29, 1993; In Final Form: June 21, 1993"
Pulsed laser evaporated A1 atoms were reacted with H2 in excess argon during condensation at 10 K. Matrix infrared spectra, deuterium substitution, and ab initio calculations identified AlH, AlH2, AlH3, and several A12H2 species. The spectra are complicated by H2 complexes with the major products. Activation energy for these reactions was provided by hyperthermal aluminum atoms from the laser ablation process. Absorptions for AlH3 increased markedly on photolysis, which facilitated their identification.
Introduction Pulsed-laser evaporation is an effective method for producing atoms to react with small molecules on condensation in excess argon, particularly for infrared spectroscopic inve~tigations,l-~ where larger quantities of product molecules are needed for detection as comparedto electron spin resonance studies.8~9Recent experimentswith pulsed-laser evaporated aluminum atoms showed that a substantial fraction of the atoms so produced are hyperthermal and they undergo reactions with 0 2 that require activation energy.5 Similar observations have been found for pulsed-laser evaporated gallium and indium atoms.6 It was therefore decided to examineother metal atom reactions that are known to require activation energy using the pulsedlaser evaporation technique. A particularly interesting case is the reaction of A1 and H2 examined earlier by Parnis and Ozin in solid krypton mat rice^.^ These workers co-deposited thermally evaporated A1 atoms with concentrated Kr/H2 = 12/1 samples and obserued no reaction. However, on photolysis with nearultraviolet light, A1 atom signals decreased and infrared bands assigned to AlH andAlH2 a ~ p e a r e d .Two ~ sets of AlHzstretching mode absorptions were reversibly increased and decreased on selective photolysis and did not appear to follow the bending mode absorption,which raises questions about the AlH2 trapping sites in the matrix in the presence of considerable excess hydrogen. Since near-ultraviolet photolysis fostered the A1 0 2 insertion reaction, which was also favored with hyperthermal A1 atoms from pulsed-laser evaporation,s pulsed-laser evaporatedA1 atoms were reacted with H2 during condensation with excess argon. Diatomic A1H was the major product in these experiments as well as several isomeric (AlH)2 species. Another new species produced in these experiments,AlH3,is of considerablechemical interest as the precursor to the elusive dialane molecule;l* the analogous diagallane (GazHs) molecule has recently been characterized.'
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Experimental Section The technique for pulsed-laser evaporation of A1 is identical to that employed in recent boron and aluminum ~ t u d i e s . ~The J aluminum target (Al, Aesar, 99.998%) was mounted on a rod rotating at 1 rpm. The Nd:YAG laser fundamental at 1064 nm with 6-Hz repetition rate, 10-ns pulse width, and 30-50 mJ/ pulse laser power was focused on the A1 target with a 10-cm focal length quartz lens. Aluminum atoms were thus codeposited with H2/Ar mixtures at 2 mmol/h for 2-4 h periods; aluminum atom concentration is estimated at less than 0.2%. FTIR spectra were recorded on a Nicolet SDXB at 2 cm-l resolution; frequency Abstract published in Advance ACS Abstrucrs. August 15, 1993.
0022-3654/93/2097-10295$04.00/0
accuracy is *0.5 cm-l. Annealing and photolysis with a 175-W mercury street lamp (Philips H39KB) with globe removed were also done. Additional experiments were performed for ultravioletvisible spectroscopic examination using a Cary 17 spectrophotometer. A similar pulsed-laser evaporation apparatus was constructed using quartz sampleoptics. In contrast to the infrared experiments, slightly less laser power and shorter deposition periods were employed.
Results
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Infrared spectra of the A1 H2/Ar system will be presented. Strong, sharp product absorptions were obtained at H2 concentration near 2% and laser power near 40 mJ/pulse. Increasing the H2 concentration to 4% led to broader bands and decreasing concentration to 1% led to weaker product absorptions. In the cases of HD/Ar and (H2 + Dz)/Ar mixtures, sharp spectra were recorded at total concentration of 4% and bands of hydrogencontainingmolecules were more intense than bands of deuteriumcontaining molecules. Blank experiments (H2/Ar mixtures without A1 and A1 atoms with pure Ar) showed that the standard impurities in this system were the COS CO, H20, Al20, AlO2, and A1202 molecule^.^ Weak absorptionsdue to AlCO, Al(CO)2, and A12C03were also observed.12 A1 + Hz. Spectra of this system after deposition and photolysis are shown in Figures 1 and 2 using 40 mJ/pulse power, and the bands are listed in Table I. Similar spectra were recorded with middle and higher laser powers. Lower laser power gave only the 1591.4-cm-l band. New bands of medium intensity and 1882.8, 1846 (broad), 1822.9,1806.6,1770.0,1668.7,1646.9,andastrong 1591.7 cm-l band were observed in the 195&1550-cm-1 region. The bands at 1882.8, 1668.7, and especially 1646.9 cm-1 were observed after deposition with higher laser power. In the 900-600-~m-~ region new bands were observed at 890, 844.1, 783.4, 766.4, 744.7, and 697.9 cm-I. Concentration dependence, annealing, and photolysis established similar behavior for the following groups of bands: 1882.8, 783.4, 697.9 cm-1 (I); 1822.7, 1774.2, 744.7 cm-1 (11); 1806.2, 1770.0 cm-l (111); 1846, 1835 cm-l (IV); 1160.9 (not shown), 844.1 cm-1 (V). The 1668.7-cm-l band was in good correlation with the 1127- (not shown) and 890-cm-1 bands (group VI). Ultraviolet photolysis of the matrix led to marked growth of intensity of the group I bands, decreasing intensity of the groups I11 and V bands, moderate growth of group I1 bands, and slight growth in the 1591.7-cm-1 band of A1H. Annealing the matrix revealed a broad band in the 1880-175O-cm-' region and broadening of bands in 800450-cm-1 region. The intensity of group I and I11bands decreased and group I1 increased. Photolysis during deposition gave primarily group I, AlH, and weaker 766.4-cm-l bonds. 0 1993 American Chemical Society
Chertihin and Andrews
10296 The Journal of Physical Chemistry, Vol. 97, No. 40, 1993
0"
0
a
a 1
d VI N
d
VI N
-,a t
a
I 1050
1000
1850
1SOO
1750
1700
WAVENUMBERS
Figure 1. Infrared spectra of Al
1050
1000
1550
CM-1)
+ H2/Ar
(1:50) system in the 1950-1550-cm-1 region. (a) After 2-h deposition with 40 mJ/pulse; (b) after full arc photolysis. Vertical slashes denote new product bands in the H20 region.
WAVENUMBERS
Figure 3. Infrared spectra of AI
n
d T
as0
BPO
780
740
WAVENUHSERS
700
soo
CM-1)
D2/Ar (1:50) system in the 145&1050-~m-~ region: (a) After 2-h deposition with 40 mJ/pulse; (b) after full arc photolysis. !! !
6 moo 4
+
1
I 800
CM-1)
Figure 2. Infrared spectra of A1 + H2/Ar (150) system in the 900-600-cm-1 region. (a) After 2-h deposition with 40 mJ/pulse; (b) after full arc photolysis.
TABLE I: Absorptions (cm-l) Observed in Pulsed-Laser Evaporated AI Atom Reactions with H2 and D2 in Excess Argon H2 Dz ratio (H/D) grouplhv(anna 1378.1 1882.8 1.3662 4++t 1.369 IV(O(+ 1348 1846 1343 1.366 IVIOI+ 1835 1.365 III+I+ 1335 sh 1822.7 1812.6 1331.2 1.3616 I1 site 1.3635 IIIt 1325.0 1806.6 1.3818 III+I+ 1286.7 1774.3 1278.9 1.3840 1111 1770.0 1.3781 VI 1210.9 1668.7 1194.3 1.3784 HAlAlH 1646.2 1176.8 1.3775 H2--AlHI+I+ 1621.1 1.3761 H~--AIHlOl+ 1165.0 1603.1 1.3745 AlHl+t 1591.4 1157.8 1.390 VI 1127 811 1.3613 V 852.8 1160.9 1.3841 VI 643.0 890.0 1.3671 V 617.2 844.1 568.2 783.4 1.3787 4++t 779.9 sh 565.0 sh 1.3804 553.9 766.4 1.3836 ?IO[+ 543.4 1.3704 744.7 IIJ+(+ 702.5 sh 517.0 sh 1.3588 697.8 513.8 1.3581 Il++t I, Band group designation, photolysis, and annealing behavior (+ for increase, 0 for no change, - for decrease). AI + Dz. Spectra of this system are shown in Figures 3 and 4, and the bands are also given in Table I. The most intense band in the spectra was 1157.8 cm-1 with low laser power. Increasing
s :o
5;s
so0
.is
si0 WAVENUMBPRS
525
.a0
4;s
.so
(CM-1)
Figure 4. Infrared spectra of AI + D2/Ar (150) system in the 650450-~m-~ region. (a) After 2-h deposition with 40 mJ/pulse; (b) after full arc photolysis.
laser power revealed new bands, which based on studies of concentration dependencies and photolysis, divided into groups corresponding to the A1 Hz system. The bands 1378.1,568.2, and 513.8 cm-1 are designated group I; the intensity of these bands grew markedly after photolysis. The bands 1331.2 and 1286.1 cm-1 are associated with group 11, 1325.0, 1278.9, and 543.4 cm-1 to group 111, 1348 and 553.9 cm-1 to group IV, and 852and617.2cm-1 togroupv. Theband 1194.3cm-lisingood correlation with 1646.9-cm-1 band. Weak 1210- and 8 10-cm-1 bands are assigned to group VI. AI HD. Spectra of this system are shown in Figures 5-7. Bands of middle intensity at 1882 cm-I, broad band width 1830-cm-1maximum, 1789.1,1771.5,1668.9,1646.9, and 1591.7 cm-1 were observed in the AI-H stretching region and a t 1378.5, 1368, 1355, 1300, 1210.2, 1194.3, and 1180.5 cm-1 in theA1-D stretching region. The strongest bands in the spectrum were a t 1591.4 and 1157.8 cm-1. Photolysis revealed sharp growth of the following bands: 1882.0,1378.5,1367.2,1355.5,781.0,716.4,652.4,646.2,568.8, and 571.5 cm-1, which is in good agreement with bands of group I. Also the intensity of bands 1789.1 and 1300.0 cm-1 decreased and the intensity of a 1797.8-cm-1 band increased on photolysis and on annealing. AJ + Hz D2. The spectra of this system were also recorded. In all cases bands of H2-containing molecules were more intense than bands of D2-containing molecules. The same bands from
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+
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The Journal of Physical Chemistry, Vol. 97, No. 40, 1993 10297
Pulsed-Laser Ablated A1 Atoms Reacted with H2
TABLE Ik Observed and Calculated Frequencies for AlHz and Hz--AlHz Complexes*
T
v3
VI
v2
1822.7 1806.6 1998.9 1797.1 1789.1 1971.4 1331.2 1325.0 1448.8
1774.2 1770.0 1945.8
744.7
~~
HyAlH2 AlH2
d
HD--AlHD AlHD D2--AlD2 AID2
t
! d
obs obs cal obs obs cal obs obs cal
1300.0 1417.5 1286.7 1278.9 1385.5
711.8 622.1 543.4 517.3
Calculated at MBPT(2) level with DZP basis set; H-AI-H angle = 118.0O; r(A1-H) = 1.558 A. (I
1950
1000
1950
le00
1650
1700
1750
WAVENUMBERS
+
1600
I 1550
