  Infrared fine-structure emission lines from trace metals are powerful
diagnostics of the interstellar medium in galaxies. We explore the possibility
of studying the redshifted far-IR fine-structure line emission using the
three-dimensional (3-D) power spectra obtained with an imaging spectrometer.
The intensity mapping approach measures the spatio-spectral fluctuations due to
line emission from all galaxies, including those below the individual detection
threshold. The technique provides 3-D measurements of galaxy clustering and
moments of the galaxy luminosity function. Furthermore, the linear portion of
the power spectrum can be used to measure the total line emission intensity
including all sources through cosmic time with redshift information naturally
encoded. Total line emission, when compared to the total star formation
activity and/or other line intensities reveals evolution of the interstellar
conditions of galaxies in aggregate. As a case study, we consider measurement
of [CII] autocorrelation in the 0.5 < z < 1.5 epoch, where interloper lines are
minimized, using far-IR/submm balloon-borne and future space-borne instruments
with moderate and high sensitivity, respectively. In this context, we compare
the intensity mapping approach to blind galaxy surveys based on individual
detections. We find that intensity mapping is nearly always the best way to
obtain the total line emission because blind, wide-field galaxy surveys lack
sufficient depth and deep pencil beams do not observe enough galaxies in the
requisite luminosity and redshift bins. Also, intensity mapping is often the
most efficient way to measure the power spectrum shape, depending on the
details of the luminosity function and the telescope aperture.
