The case is considered of a weak Gaussian harmonic pulse of frquency (2sϩ1) passed through an atomic medium collinearly with an intense Gaussian laser pulse of frequency . The harmonic beam may be amplified or deamplified according to the balance between stimulated emission and absorption. A simple analytic expression for the gain is derived in the tunneling limit of ionization. It predicts a positive ͑negative͒ gain for positive ͑negative͒ delay of the harmonic pulse with respect to the laser pump pulse. The gain may be significant and proportional to the delay for delays of a few laser periods, much shorter than the pulse envelopes. Possibilities for experimental observations are briefly discussed.

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High-resolution electrical resistivity data ( p , d p / d T ) revealed a fascinating succession of fluctuating modes and magnetic phases in Gd,_,Y, crystals at the onset of helimagnetism (x, -0.3). An improved magnetic phase diagram is proposed uhich contains two multicritical points near x,. Our data refute the suggested existence of a Lifshitz point in G d , -xY, at ,Y-0.3. i.e. the coexistence of ferromagnetic. paramagnetic and helimagnetic (q+ 0) phases.

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