Procesos de preparación de los envases de aluminio para el proceso de fundición

VMI is utilized to study the dissociation dynamics of CH2OO following UV

excitation on the B1A′ - X1A′ transition. Previously, the O 1D fragments associated with the lowest spin-allowed O 1D + H2CO X1A1 channel were imaged following excitation at

308, 330, and 360 nm.25 In this chapter, the O 3P fragments associated with the higher spin-allowed O 3P + H2CO a3A channel are examined following excitation at 330 and

Figure 2. Top: Representative velocity mapped raw image recorded with vertical polarization (arrow) for the UV pump laser at 330 nm in the plane of the detector. Middle: Total kinetic energy release (TKER) distribution for O 3P2 products resulting

from photolysis of the simplest Criegee intermediate CH2OO at 330 nm. Bottom: TKER

distribution for O 3P2 products resulting from photolysis of CH2OO at 350 nm. The

arrows in the middle and bottom panels indicate the termination of the TKER

distributions. The sticks in the bottom panel illustrate a model for vibrational excitation of the H2CO a3A co-fragment in the CH2 wag (ν4, blue) and C-O stretch (ν2, purple)

modes (see Table 1). (Simulation of rovibrational excitation of the H2CO a3A co-

fragment is shown in Figure A3.52_{) The molecular structure illustrates the angle} 

between the transition dipole moment (TDM, ) of CH2OO and the recoil velocity (v)

350 nm. The O 3P2 fine structure state is found to be strongly preferred following UV

photodissociation of CH2OO. As a result, the present experiments focus on the O 3P2

product state exclusively. The two-dimensional ion image of the O 3P2 fragments

obtained following UV excitation of CH2OO at 330 nm, near the peak of the B-X

spectrum, is shown in Figure 2. The radial distribution is integrated over the angular coordinate to obtain the velocity distribution for the O 3P2 fragments. The total kinetic

energy released (TKER) to the H2CO a3A + O 3P2 products is then obtained using

conservation of linear momentum based on the velocity distribution of the O 3_P 2

fragments. The resultant TKER distribution shown in Figure 2 is quite broad and unstructured with a peak at ~1900 cm-1 and breadth of ~2200 cm-1 (FWHM); a

polynomial fit is superimposed on the data as a guide to the eye. The TKER distribution obtained following UV excitation of CH2OO at 350 nm is also shown in Figure 2 (bottom

panel). Again, the TKER distribution is broad and lacking structure, while peaking at a lower energy (~850 cm-1 _{with FWHM of} _{1100 cm}-1_{) due to the decreased energy}

available Eavl to products.

Using conservation of energy, Eavl can be expressed as

Eavl = Ehv D0 ΔE = TKER + Eint(H2CO)

where Ehv is the photon energy, D0 is the dissociation energy of CH2OO X1A′ to H2CO

X1_A

1 + O 1D products, and ΔE is the energy spacing (26.7 kcal mol-1) between the two

product channels, H2CO X1A1 + O 1D and H2CO a3A + O 3P2. The internal energy of

CH2OO X1A′ is not included because it is negligible after cooling in the supersonic

the TKER distribution, or internal (vibrational + rotational) energy of the H2CO a3A co-

fragments, Eint(H2CO). The highest kinetic energy in the TKER distribution, ET,max, will

correspond to the lowest internal energy of H2CO, which is assumed to be the zero-point

level of the excited a3A electronic state. The energy available to the products, Eavl, is

thus equivalent to ET,max. The termination of the TKER distribution at ET,max ~ 3900 

120 cm-1 upon 330 nm excitation can be used to establish the threshold for the higher energy H2CO a3A + O 3P2 product channel of ca. 76 kcal mol-1 (Ehv ET,max = D0 + E);

this corresponds to UV excitation of CH2OO X1A′ at 378 nm. Subtracting the energy

spacing between the two product channels then yields an upper limit for the dissociation energy of CH2OO X1A′ to H2CO X1A1 + O 1D products of D0 49.0  0.3 kcal mol-1.

Repeating the evaluation based on 350 nm excitation of CH2OO X1A′ and termination of

the corresponding TKER distribution at ET,max ~ 2100  130 cm-1 yields essentially the

same upper limit. This value is compared with a previous experimental determination and theoretical predictions in the discussion section.

The anisotropic angular distribution of the O 3P fragments (Figure 2) makes it clear that photodissociation of CH2OO at 330 nm is prompt, and rapid compared with the

ps timescale for rotation of the Criegee intermediate. A more detailed analysis of the angular distribution of the O 3P2 fragments enables the orientation of CH2OO B-X

transition dipole moment (TDM,) with respect to the recoil velocity vector ( v ) of the O-atom to be determined. The lab frame angular distribution, I(θ), can be expressed as

 

where θ is the angle between the recoil direction and the polarization of the UV photolysis laser, P2 is a second-order Legendre polynomial, and β is the anisotropy

parameter. The angular distribution of the O3P2 products exhibits a β parameter of 1.3(1)

near the peak of the TKER distribution. The β parameter increases slightly with kinetic energy over the FWHM of the distribution. In the molecular frame, the β parameter is related to the angle χ between the TDM and v via 2 P₂(v) 2 P₂(cos ) .44 Accordingly, the experimentally derived β parameter yields an average angle χ of 29(2)°. This value is compared with a previous experimental determination, obtained when detecting O 1D products, and a theoretical prediction in the discussion section.25