1.5 Mb MPEG movie of radial wave packets in H and Li
The bottom box plots the electron's radial probability distribution
in Li and the upper box is in H. The arrow on the top is a clock that gives
the time in units of the classical period of motion; at t=0 the arrow points
up. The classical period at the energy of an n=40 state is 9.7 picoseconds.
Every time the arrow points up the time is a multiple of the period whereas
when it points down the time is a "halfinteger" period. Since the electron
starts near the nucleus you should find that every time the arrow is up
the electron probability is near the nucleus whereas when the arrow points
down the probability is at large distances (as in the image above).
There are a number of interesting features that deserve comment. One
of the things to notice is that the relatively high speed of the electron
in the radial range 0-1500 atomic units compared to the range 1500-3000
means that whenever the probability enters the small r region the height
of the packet decreases and the radial width increases. Also, whenever
there is interference between inward and outward moving pieces of the packet,
the interference pattern has a much smaller wave length in the small r
region than in the large r region. Also, notice that the two wave packets
disperse very quickly, after only 2-3 radial periods the wave packets cover
almost the whole classically allowed region. Dispersion is a wave phenomenon
that arises when the group velocity of a wave changes with the frequency
of the wave. The dispersion can also be related to properties of the classical
motion: if the classical period is energy dependent then the wave packet
will disperse if you excite more than two eigenstates. For the Coulomb
potential, the period of motion increases with the energy (by analogy remember
that the year for Mercury is shorter than that for Earth is shorter than
that for Jupiter is shorter than that for Pluto). Thus, these radial waves
are dispersing by having their higher energy components move to the back
of the packet.
One of the final interesting features is the similarity between the
H and Li results. This might seem somewhat surprising when you realize
that the core electrons in Li cause the l=0 energy levels in Li to shift
so much they are nearly halfway between the energy levels in H; thus making
them almost as dissimilar as possible. The reason for the similarity is
that to lowest order the energy levels are all shifted by the same amount.
It is only the change in this shift with the number of nodes that will
be apparent in the wave packet. Thus it is necessary to have the packets
evolve to somewhat long times before the differences become manifested.