  We analyze new optical spectra of a sample of 502 galaxies in close pairs and
n-tuples, separated by <= 50/h kpc. We extracted the sample objectively from
the CfA2 redshift survey, without regard to the surroundings of the tight
systems. We probe the relationship between star formation and the dynamics of
the systems of galaxies. The equivalent widths of H\alpha (EW(H\alpha) and
other emission lines anti-correlate strongly with pair spatial separation
(\Delta D) and velocity separation. We use the measured EW(H\alpha) and the
starburst models of Leitherer et al. to estimate the time since the most recent
burst of star for- mation began for each galaxy. In the absence of a large
contribution from an old stellar population to the continuum around H\alpha,
the observed \Delta D -- EW(H\alpha) correlation signifies that starbursts with
larger separations on the sky are, on average, older. By matching the dynamical
timescale to the burst timescale, we show that the data support a simple
picture in which a close pass initiates a starburst; EW(H\alpha) decreases with
time as the pair separation increases, accounting for the anti-correlation.
This picture leads to a method for measuring the duration and the initial mass
function of interaction-induced starbursts: our data are compatible with the
starburst and orbit models in many respects, as long as the starburst lasts
longer than \sim10^8 years and the delay between the close pass and the
initiation of the starburst is less than a few \times 10^7 years. If there is
no large contribution from an old stellar population to the continuum around
H\alpha the Miller-Scalo and cutoff (M <= 30 M_\sun) Salpeter initial mass
functions fit the data much better than a standard Salpeter IMF. (Abridged.)
