In a small saucepan, bring 1/2 cup sugar, 1/4 cup water, and lemon juice to a boil, stirring to dissolve sugar, 1 to 2 minutes. Let cool.
In a small bowl, combine jam with 3 cups raspberries, mashing slightly. In a large bowl, whip cream and 2 tablespoons sugar to stiff peaks.
Slice pound cake 3/4 inch thick; brush both sides of slices with lemon syrup.
Fit 1/3 of slices snugly in the bottom of a 4-quart trifle dish or other glass bowl, trimming edges if necessary. Gently spread top of layer with 1/3 of raspberry mixture, and then 1/3 of whipped cream. Repeat to make two more layers; garnish with remaining cup raspberries. Refrigerate until ready to serve, up to 24 hours.