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region. They consist mainly of three types, when
rotationally resolved: simple ðX0 ¼ 0Þ ðX00 ¼ 0Þ
transitions with no Q-branch; ðX0 ¼ 1Þ ðX00 ¼ 0Þ
transitions, with typical P, Q, R structure, and
ðX0 ¼ 2Þ ðX00 ¼ 2Þ transitions (P, Q, R structure,
but with R(0), R(1), P(2), Q(1) lines missing). The
first two sets of transitions have been shown [12] to
originate from several of the well-characterized vi-
brational levels of the MgO(X1Rþ) ground-state [1–
3], and the thirdset [12] from vibrational levelsof the
lowest-lying metastable triplet state, MgO(a3P2)
[4–6]. Many of the spectra are beautifully well-re-
solved, and are almost Ôtext-bookÕ examples of these
kinds of transitions. The spectra reported here are of
the ðX0 ¼ 1Þ ðX00 ¼ 0Þ type, and have been shown
to be due to the F1P1 X1Rþ electronic transition
of the 24Mg16O molecule. Several rotationally re-
solved bands are consistently assigned to the F X
transition, and we confirm SinghÕs (0,0) band as-
signment [7]. The vibrational and rotational con-
stants we have determined for the F1P1 state are
also qualitatively consistent with Peyerminhoff and
co-workers [11] notion that this state has ÔmixedÕ
valence–Rydberg character.
which act in concert to ionize the neutral species.
Resonance-Enhanced Two-Photon Ionization
(R2PI) spectra are obtained by frequency scanning
the output of one of the two dye lasers while
holding the other dye laser constant in frequency,
monitoring the MgOþ signal in the mass spectrum.
The ultraviolet radiation used for the resonant step
in the two-photon ionization process was obtained
by non-linear frequency doubling (KDP or BBO
crystals) of a dye laser output. Some single-dye-
laser spectra were also taken where 355 nm radi-
ation from the YAG laser which pumped the dye
laser was used for the ionization step.
3. Results and discussion
We have recorded five vibrational bands of the
F X transition of 24Mg16O with sufficient signal-
to-noise for accurate rotational analysis. Shown in
Fig. 1 is the (1,1) band (of 25Mg16O to illustrate the
excellent signal-to-noise of the spectra (25Mg has
only 10% abundance!)). Because the B constants of
the X1Rþ and F1P1 states are so similar, Dm ¼ 0
transitions are very strong, due to the Franck–
Condon principle, and Dm ¼ 0 transitions are very
weak, unlike the MgO(E1Rþ X1RÞ electronic
transitions [12] or electronic transitions [12] from
the MgO(a3P2) state.
2. Experimental
The experimental apparatus has been described
in detail elsewhere [12,13]. Briefly, 532 nm radia-
tion from a Molectron MY-32/10 Q-switched
Nd:YAG laser is focused onto the surface of a
rotating pure Mg rod which is inside a 100 l vac-
uum chamber (operating pressure ꢁ5 ꢂ 10ꢀ5 Torr)
and is slightly beyond the 2-mm exit hole of a gas
source. The laser-vaporization products are en-
trained in a gas pulse produced by a general valve
backed by 40–100 psi of helium or neon gas con-
taining 1% N2O. Mg species from the discharge
react with the N2O in the gas pulse to form MgO,
and the ensuing supersonic expansion cools the
MgO molecules to rotational temperatures of
ꢁ5–15 K, depending on the expansion conditions.
The beam traverses a 60 cm region through a
skimmer before entering the ionization region of a
one meter time-of-flight mass spectrometer. Here
the MgO molecules are interrogated with the
outputs of two simultaneously pumped dye lasers,
Shown in Table 1 are the transition wavenum-
bers and B00; B0 rotational constants of 24Mg16O
resulting from our rotational analyses. Our tran-
sition wavenumbers for the band origins of the
Fig. 1. High-resolution spectrum (top) of the F–X (1,1) band of
the minor isotope 25Mg16O; computer simulation (bottom):
T ¼ 10 K; B00 ¼ 0:550 cmꢀ1; B0 ¼ 0:538 cmꢀ1
.